Left 4 Dead/WMG

Possible new Infected for Left 4 Dead 3

 * The Swinger: A hooting orangutan-like Infected, who frequents ceilings and general high places. It lies in wait just above the Survivors' heads, only to suddenly swoop down, pick up a victim, and fling them into the air. The Survivor is completely immobile when in the air, and takes serious injuries upon hitting the ground. There is a 2-second period, however, between the grab and the throw, where the Swinger can be killed.
 * The Crawler: A tiny and tenacious Infected, vaguely resembling a child. It's able to scale walls, walk upside down, and be unfazed by pipe bombs, but is the single weakest creature in the game. Unlike other Special Infected, however, it has obscene numbers on its side; they usually attack in groups of 10 or more.
 * The Boogeyman: A Special Infected completely devoted to the art of psychological attack. If a Survivor tries to go it alone, the Boogeyman will lock doors, close windows, and shut off all sources of light. The Boogeyman's sole goal is to disorient the Survivor, and keep him/her scared for as long as possible, through several smoke-and-mirror tricks and sheer inability to hit. However, it possesses the durability of a normal Infected.
 * I somewhat like this idea, myself, but survivors aren't very likely to split up by themselves.
 * The Leech. He attaches to your back, just like a jockey, but you can still shoot other things, and the camera doesn't even pan out to show that he's there. However, the screen will have an orange tint to it and your health will slowly ebb away. You cannot get the Leech off yourself, someone else has to shoot it for you.

Left 4 Dead 3 will be set in Britain, 4 weeks after first infecion
Because not only would fighting the Infected in the ruined streets of London be awesome, but it would be 28 days after the original outbreak, and Valve can't pass up a reference like that.
 * I would give a lot to fight MP zombies in the House of Commons...
 * I love this idea.The last finale could involve holding out in a pub waiting for the army to arrive.
 * I'd kill to see a Canadian survivor in the middle of this.
 * And if there's a sequel to The Passing, Francis will hate him immediately.
 * One of the playable survivors MUST be an actor. A shakespearian one, preferably.
 * This wouldn't really work unless you like using Melee weapons and nothing else the majority of the time. Sure, "Everyone and their Mums" might have guns in Sandford, but if it was set in London the only firearms would be the obligatory Winchester found inside a Pub, and the guns issued to special branches of the Police Force.
 * Although, thinking about it, the Civil Nuclear Constabulary has access to some pretty major firepower. And it would definitely allow for some advancement in weapons as you go through the game. Early on? Melee, maybe a Shotgun. Defend a Pub midway through the game? Winchester for one of you. Find a Police Station, grab some better weapons. Find the Civil Nuclear Constabulary? Shoot EVERYTHING.
 * The problem with that point is it's a Zombie Apocalypse. It wouldn't be too much of a stretch to imagine that the British Army would be doing what they could to fight off the zombies, or that a few most of them wouldn't make it, or that the players would be able to grab a few guns off of them. Hell, the first two games have US Army-issue M16s and M1014s all over the place, why wouldn't a sequel set in Britain have dropped L85s and L128s show up frequently?

Insert New Settings/Survivors Here

 * Buffy, Faith, Angel and Xander.
 * The Midnight Riders as they run a parallel route to the Left 4 Dead 2 survivors. Hinted at by their bus being seen speeding past in The Passing.
 * Michael, Sam, Fiona and Madeline make their way across Florida, starting in the Miami Suburbs.
 * Jon Stewart, Stephen Colbert, Conan O'Brien and Jay Leno battling their way through New York, with Conan and Leno snarking at each other the entire way.
 * Keith, Shaun, Yahtzee and some guy named Rodger fighting their way from London to Antarctica for no apparent reason because.... well it would be AWESOME.
 * Stan, Kyle, Cartman, and Kenny all grown up. Just... just 'cause.
 * Columbus, Tallahassee, Wichita, and Little Rock. 'Cause someone had to say it.
 * Jerry, George, Elaine and Kramer. Possible locations include Jerry's Apartment (with an air conditioner to drop on zombies climbing up the building), the Chinese Restaraunt, and the Parking Garage. Pills are replaced with Junior Mints, as they're very refreshing. Only one tank per level, and it always says "Hello, Jerry!" before attacking.
 * What's the deal with zombies anyways?
 * You know, my friend, Bob Saccamano, he knows this guy that knows a guy on Fifth Avenue that sells legit boomer puke...
 * Greg, Ryan, Colin and Wayne (or insert your favorite guest star here.) Because it would be awesome.
 * Keith, One Member of the Midnight Riders, a street punk, and a nurse.
 * Phoenix, Maya or Franziska, Edgeworth, and Gumshoe. The latter of whom would reach incomprehensible levels of woobiedom due to his fear of blood.
 * I reckon you should drop Gumshoe, make it Maya and Franziska. I could see Maya with an Auto-Shotgun, Phoenix with a Hunting Rifle and the Prosecuters with Assault Rifles.
 * Chuck Norris, Mr/T, Mr. Zurkon, and Jackie Chan.
 * Smitty of the Midnight Riders, Keith, Chicago Ted and Nick's ex-wife.
 * A prequel to the first game, starring four survivors: A hooded traceur, an overweight alchoholic, a tall chain-smoker, and an easily upset woman. The ending is less than happy.
 * Is this the best idea? It might be.
 * Japan, starring a generic angry teen who likes breaking things apart, his tsundere girlfriend with a spear and massive boobs, a young samurai lady with a disturbing murder fetish, and a fat gun otaku.
 * A prequel taking place in antiquated Britain, only with vampires instead of the usual Infected, with a few non-copyrighted monsters serving in place of the Special Infected. The 4 survivors here are famed authors taken from time: Sir Arthur Conan Doyle, Howard Philips Lovecraft, Herbert George Wells, and Ayn Rand.
 * JD, Elliot, Turk, and Carla making their way through Sacred Heart hospital after the infection. Dr. Cox and the Janitor as super special secret unlockable characters. And there'd be an excuse for pills, adrenaline shots, and painkillers to be lying everywhere...
 * Chuck Norris, The Most Interesting Man in the World, The Man Your Man Could Smell Like, and Mr. T. Those zombies are screwed... Memetic Badasses unite!
 * A gangster, a police officer, a doctor, and a nun. Okay, so it's not AS cool as all the others listed, but still…
 * Britian. A cop, two soldiers, The Pyro, an- Wait, never mind.
 * Slowbeff, Diabeetus, Proteus and Dave_o.
 * Nicholas Angel, Jules, Ray and Devon
 * Angel leads with the Automatic Shotgun, Jules and Ray run crowd control with Assault Rifles and Devon is the chief-rescuer with a Hunting Rifle
 * Ray is very protective of Devon while Jules and Angel compete to be the most Big Damn Heroic
 * Notable moments include Devon saving Ray from a Witch and Jules going Back-to-Back Badasses with Angel
 * Pyro, Sniper, Engineer, and Scout. They ran out of ammo for their weapons ages ago...
 * In short, any four of the nine MAY do.
 * David Draiman, Hayley Williams, Eminem and Jack Black
 * Russia, some time after the first outbreak. Josef, a street punk, Boris, a Ukrainian oil executive, Natalya, a jewelery store clerk, and Yuri, aging ex-KGB agent. They fight their way out of Moscow, through the wilderness and St.Petersburg to reach Archangelsk and escape on a decommissioned submarine with the help of some navy types. Would involve defend-a-moving-vehicle crescendo featuring a new Special Infected: The Runner. May also involve an end Boss Fight with a hideously mutated super-tank: The Destroyer.
 * Vulcan Raven, Sniper Wolf, Revolver Ocelot and Psycho Mantis. The game takes place before they get involved in the whole Shadow Moses mess. Alternatively, place the game some forty years into an alternate future and the heroes are the B&B Corps. The zombies are basically screwed either way.
 * Any group that includes Chicago Ted.
 * Dutch, Revy, Rock, and Benny. When there's no more room in Hell the dead settle for the next best thing.
 * Aovnick, Archaon, PuppySky, and Djy1991.
 * baldurk, EthanSteele, cKnoor, and Blister.
 * Caleb, Ophelia, Gabriel, and Ishmael. Preferably with Jojo the Idiot Circus Boy replacing one of the Special Infected, and a Sawed-Off Shotgun in place of the pump shotguns.
 * Marv, Hartigan, Dwight, and Gail. The Infected show up in Basin,but they picked the wrong people to fuck with.
 * Calypso, Needles Kane, Krista Sparks, and Carl Roberts. The whole thing was Mr. Grim's fault.
 * Doctor Weird, Snoopy, Tigger, and Sharona Fleming for Rule of Funny.
 * Rorschach, Violet, Sora, Dante, and Babydoll for a Left 4 Dead version of The League of Extraordinary Gentlemen.
 * Buck, Dutch, Romeo and Mickey. And they thought the Covenant was bad!
 * Stephen Fry, Alan Davies, Morgan Freeman and the Doctor. One of the finales will be saying a predictable answer to get the attention of a helicopter and one will be waiting for a severely damaged TARDIS to materialize.
 * A no-nonsense librarian who is the team leader (and has special connections), an farm girl from a big family, a fashion designer with a keen eye, a competitive and fast athlete, a confectioner and baker (who can be a bit of a cuckoo), and an animal caretaker who is quicker in healing. Remind you guys of anything? Hey! It isn't any more crazy than all the above guesses, is it? Well, maybe it is, but this is the WMG page. ANYTHING can be put in.
 * Chuggaaconroy, Nintendo Capri Sun, Proton Jon, and Josh Jepson. Just because. THERUNAWAYGUYS!
 * A priest, a rabbi, a Mormon and a duck walk into a bar... Wait. Wrong page.

There will be 2 more Left 4 Dead games, making a total of 4
The exact nature of this theory goes as follows: The Left 4 Dead series takes place in the United States. The first game took place on the East Coast. The second one took place in the deep Slouth. The next two are going to cover the rest of the country - Game 3 is going to cover the Northern border, with at least one stage taking place near Mt. Rushmore, and generally just more Zombie antics, with maybe a little development towards figuring out the exact nature of the disease. But game 4 is going to have a lot more plot, with a CEDA worker and a military personel being part of the West Coast team, and the ultimate goal is not going to be survival, but containment. Keeping the disease contained within the states and ensuring no infected make their way to any other continent. It will be a Downer Ending as all of the USA is overrun, but the glimmering hope will be that the rest of the world is safe, at least...until the cutaway where we see one Zombie purposefully walk into the ocean, followed by more...
 * You know Valve can't count to three. The most we'll see is more L4D2 DLC.

New Survivor Speculation
(Note: Feel free to add to this list with any idea you have.)

Known Characters:
 * A Member of The Midnight Riders, likely lead vocalist Smitty or bass player Jake. Would frequently make references to their songs and use a guitar as their weapon of choice.
 * Keith who would tell "My Buddy Ellis" stories and inform zombies that "Unless your gonna drown, burn, cut and tear gas me you sons'a'bitches ain't got shit!"
 * Chicago Ted, killing zombies cause it's a mission from god, will also make use of his own Badass Creed reminding every damn infected that "No Zombie is Safe from Chicago Ted". As mentioned above, he will also be black.
 * So he's Barack?
 * Chicago Ted being a memetic mutation of himself, always refering to himself in the third person.
 * Virgil, the boat guy, teamed up with some of the survivors he rescued when his boat ran out of fuel. He'd likely be a level headed Team Leader.
 * Whitaker The Gun Shop Guy, That Six Pack won't last him long. Would be a Crazy Awesome Cool Old Guy in desperate search of Cola.
 * Nick's Ex-wife. No one knows what she's been up to. Could be interesting should she meet her Ex-Husband. Much screaming and Jerry Springer-esque shenanigans would ensue I imagine.
 * The Angels of Death mentioned in multiple graphiti.

New Survivors:
 * A college kid, separated from his girlfriend who got rescued.
 * A Mafia don, sharp dressed overcoat wearing type. Possibly related to the above Francis/Nick Gang WMG.
 * A Chinese Black Belt, Old Master and Cool Old Guy.
 * A Spanish priest in the same vein as Father Grigori.
 * A nerdy Otaku teen, who sees the apocalypse as a chance to increase his e-peen.
 * So like Zoey, but even nerdier and with a Freudian Excuse?
 * A rookie cop who's first day on the job just happened to be the day a zombie apocalypse started. Unlucky Everydude, full stop.
 * Leon?
 * A British Man who was in America on business when the infection broke loose., Sophisticated As Bloody Hell.
 * An Elderly Woman who sees fit to show these damned zombies some damn manners.
 * Coach's grandma? Yes, please!
 * A bum, the way Louis was originally designed in concept art.
 * Personally I'd like to see a goth chick, either in the Perky Goth variety, or a Deadpan Snarker. Or both!
 * How about an Iraq veteran who was the only survivor of their military outpost. Met up with the other three when they were scavenging weapons. Probably female, and probably snarky/Action Girl from being in the military.
 * A weird rich guy who spent a lot of time "preparing" for the whole thing. He'll spend the whole game in a suit of armor. He'll never show his face.
 * Batman? In a zombie game? I WANT!
 * A politician. Not be a stuck-up wuss, though. One who doesn't take no crap from no zombies.
 * "This? Scary? You've obviously never been involved in politics."
 * A weapons nut/wannabe action hero, a la Dwight from The Office.
 * Some woman of ill repute. Because even hookers have to deal with zombies. This may turn out to be Nick's ex-wife.
 * A Psychiatrist lady. She tries to rationalize the whole thing, and also tries to keep her team's morale up. If not a Psychiatrist, then a for-real doctor lady.
 * An Indian person. As in, from India.
 * An Actor or Actress, who keeps thinking that they're in a movie shoot.
 * A former Hollywood stuntman.
 * A fast food worker of poor hygiene and poorer regard.
 * Team Gibbs.
 * How have we gotten this far without any Asians? We could have a Korean store owner, a Yakuza or a Chinese tourist.
 * Marv from Sin City...well a guy can hope, can't he?
 * Female EMT (Emergency Medical Technician) who will take on the role of "Complainer".
 * Someone's grandmother.
 * Me. Because that would be freaking awesome.
 * Depends on who "Me" is.
 * An ordinary guy who likes girl's stuff. What?
 * A librarian who has relations to some of the higher-ups and turns out to be a telekinetic.
 * A farmer's daughter. Who prefers the shotgun.
 * A fashion designer who has also spent some time at the gun range, preferably with the sniper rifle.
 * A triathlete who's quick on their feet, and usually ends up needing to have to be pulled back so as not to run off on their own.
 * A babysitter/confectioner/baker who goes crazy upon getting their hands on a chainsaw, and often tells completely pointless stories about their friend nicknamed "Oatmeal".
 * A veterinarian who has a difficult time in adapting to the apocalypse, and constantly struggles to be of "credit to team".
 * A mailman who is competent in killing zombies, in spite of their eye condition that hampers their vision. It has no effect whatsoever on gameplay, but it makes them look silly.
 * A lawyer/law student/judge who makes terrible puns involving legal terms.
 * A wannabe author/musician/artist who gets inspired.
 * A soccer mum.
 * A teacher.
 * A nerdy, sociopathic teenager with Asperger's Syndrome, who tends to lament the fact that (in his opinion) he has absolutely no chance of getting a girlfriend...and gleefully welcomes the zombie apocalypse as a way to keep his mind off that fact.

New Groups:
 * Keith, Chicago Ted, Virgil, and probably a Goth Chick. Chicago Ted would probably enjoy Ellis stories.
 * Replace "Goth Chick" with "Nick's Ex-Wife" and you got yourself a dream team right there.
 * The Mafia Don, a Bum, Iraq Veteran, and the President.
 * Nathan Fillion, Jon Stewart, Seth Green and a reality displaced Xander Harris.

In Left 4 Dead 3, all the survivors will be African.
The African-American population in Left 4 Dead 2 has doubled since Left 4 Dead 1: First there was just one (Louis) now there's two (Rochelle and Coach). If the doubling pattern continues, then in Left 4 Dead 3 all the characters will be African. It might even take place in Africa, if Valve are feeling adventurous. Imagine fighting hordes of zombie Somoli pirates.
 * And they'll bump into Chris and Sheva during their adventures.

The Sequel takes place on a cruise ship one year after the first infection.
In this span of time, Rochelle has gotten pregnant and can't join the rest of her friends on "scouting missions". However, she remains on the cruise ship (Let's call it the Quadrille) and assign groups of four to the mainland to recover supplies, fuel, or just to kill more zombies. The players can now choose from 16 characters (three from the first game, three from the second, and a bunch of other people they "picked up on the way", the six we recognize have all changed in appearance) to place in the group of four to head on land. Based on who is chosen by the player (or Rochelle, who will pick randomly if you don't pick for her) the dialogue with the other characters is different. Rochelle stays in contact with the away team via radio, and gives them their objectives, interacts with the characters, and sometimes even complains about her pregnancy (She never gives birth in the game, because that would just be cliche). The gang travel up the atlantic coast, and back into Philly, where they try to find the original cause of the infection and why they were immune.

The next game will be in an entirely different genre.
Join up with thousands of other survivors in the L4D MMO! Command CEDA forces or legions of Infected in the L4D RTS! Climb onto Bill's jeep or Francis' bike and speed through the zombie-clogged streets of Philadelphia in the L4D kart racer!

The next game will actively encourage each player to screw over the rest of the team.
And it'll be called Out 4 Myself.

Everything that's happened in Left 4 Dead is just a terrible nightmare dreamt by Bill
During the scene in the Sacrifice comic where the surgeons were putting sleep drugs on Bill two days after the infection, it could have been possible that Bill hallucinated when he saw one of the surgeons turn into an infected, and once he blacked out he was in a dream the entire time during the events of Left 4 Dead all the way up until his death in the Sacrifice. The other survivors Zoey, Francis, and Louis could have been dreamt up based on actual members from his missing family. How Bill got stuck in such a long and nightmarish dream is because the Green Flu got to him during the operation (Which turned out to be nothing more than a very powerful virus that people over exaggerated on before it hit Philadelphia). Doctors eventually found a cure for the flu and cured everyone infected with it, including Bill. After his "death" in the Sacrifice, Bill wakes up in the hospital. Another ending could have been that the flu managed to kill Bill in real life too at the same time he died in his dream.

There's a reason there's no children in Left 4 Dead
I've already added this to Fridge Brilliance and the main page, but I'll put it here as well, so there's no dispute:

Zombified adults in this game can run like mad. Children cannot run as fast as an adult in full sprint. Zombies eat human flesh.There's a good reason you never see any children in these games.
 * Children are not often left alone or could fend for themselves in this situation, completely valid.

I would also have to argue that if you were a parent and saw people killing each other and were worried that might happen to your child, you might be driven to killing them yourself to save them from the horror.
 * I thought the reason there was no children was because they weren't strong enough to survive the infection, thus never turned. Same goes with the animals, which would explain all the dead cows (that or the Infected just eat them.)

The infected will eventually mutate to the point of death
The infections from Left 4 Dead have advanced to a gruesome point in Left 4 Dead 2. It appears that each infected types will just die after a certain point in time. With any other infected type is hard to tell how they change over time because they have remained the same or just been introduced.
 * Smoker: Their cancerous parts will eventually overtake them and kill them.
 * Boomer: They will eventually expand until they die.
 * Hunter: Their skin will eventually degrade until they can no longer survive.
 * Tank: Their muscles will eventually crush their entire head.
 * Here's some more.
 * Spitter: Acid will eventually dissolve their body.
 * Charger:Arm will eventually grow so large that it can no longer stand upright, and will just fall over and starve to death. (Maybe the infected in the games don't need to eat, but I've never been told that, so I'll assume they do.)
 * Alternatively, infected in general simply have short lives due to being infected with a virus. We've seen common infected walk around vomiting and in some cases, falling over and dying. It's very likely that the infected just die off regardless because of all the damage the virus does to their body.

The AI Director is House
Hence the sadism and love of pills.
 * A sadistic and almost divine being only letting you live to see you suffer? I know another House like that.

The Midnight Riders will be the Player Characters of Left 4 Dead 3
There's four of them, they're STRONGLY implied to be still alive and therefore immune (graffiti on walls here and there, their conspicuous complete absence in the Concert finale, the tour bus in The Passing), and they already have character designs (banners in Dark Carnival). It would tie in to Left 4 Dead 2 nicely to put the players in their shoes.

Left 4 Dead is a series of novels
...Converted into films. The posters, the "credits", and the AI Director all add credibility to the "film" part, but on the Hard Rain poster, it says on it somewhere "Based on the novel by Edward Ducatel". Ducatel happens to be the name of the sugar co. in the campaign. Harmless naming, or AUTHOR INSERTION? And considering how much Gabe Newell loves his mind screws...

Keith is black and the "ghost or homeless person" who stabbed him was a KKK member
I feel really bad for thinking up this WMG for stereotyping the American south, but what else would a homeless guy be doing in a robe with eyes poked out of it?
 * Alternatively, Keith COULD be Jewish, or gay (interpret that and the Ellis friendship however you want, I'm not implying anything), as the KKK don't like them either (don't ask me how I know that...).

Keith is dead, and Ellis knows it
The way Ellis talks about him seems akin to hero worship, or at least rose-coloured glasses (even if those rose-coloured glasses show third degree burns). The way all the stories seem to end in Keith's indestructibility, it would seem poetic for them to be Ellis' way of keeping Keith alive. Imagine Keith succumbing to infection, and being shot by Ellis. What better way to preserve Keith's toughness than denial?
 * Ellis says to Nick that Keith was one of the first people to be evacuated via helicopter. Then again, given the games' previous helicopter pilots...
 * Exactly! Given the series history of helicopter pilots, Keith is almost definitely dead.
 * The way Ellis speaks so highly of him could be defined both ways. Either Keith is too epic to be killed by a zombie infection or, as OP suggests, is Ellis is trying to remember his friend in his greatest moments.
 * Not "all" the helicopters crash. Until they release another Left 4 Dead 2 campaign, saying that the last one did.
 * As of the Beta of Cold Water, yes they do. (At least one very similar to the evac vehicle from The Parish).

Having "rescued" the survivors, The military will attempt to kill them.....
....But the Carriers will kill the military. Seriously, these guys can take massive amounts of damage. Being slashed repeatedly, being doused with acid, being tossed across parking lots, getting mobbed and beaten, getting clawed in the back, getting set on fire, getting blown up by propane tanks, and being nearly immune to gunfire(depending on the difficulty). They can also detect other survivors through walls. Clearly, normal human soldiers are not a match for this kind of adversity. As per the hundreds or even thousands (the latter probably being an inflated statistic) of Carriers killed by the Government, it's possible they did so using aircraft. That's why the bombing in the intro was so danger close, they are trying to kill all of the infected, Carriers included!
 * How about you try Realism + Expert? A couple hunter claw slices, and you are down. Five punches from common infected, and you are lying on asphalt crying for help. One Tank punch and you are DEAD.
 * Some of those things are just for gameplay, while normal Humans are more durable than they get credit for. They wouldn't be more of a problem than your average survivors.
 * Maybe whatever it is that prevents our heroes from succumbing to the infection also has the side effect of having them be Made of Iron in comparison to other humans.
 * that's the best theory I've heard in a while. Alternatively, Their immunity is partial-- they gain the zombie's resistance to damage but retain higher thought and social capabilities
 * Zombies aren't any more resistant to damage, they just shrug off any non-lethal wounds. If you shoot a common and then left him, he'd die as soon as he ran out of blood
 * I only meant-- that they can keep functioning despite horrific wounds until they are healed (as in "regenerate with the minimal help a first aid kit would provide"). The Partial Infection theory also explaines how they run miles at a time, beat back several people at once, and fire powerful weapons (in the case of the Magnum, one-handed) without being sore from the recoil- mixing types of special infection makes a specimen approaching superhuman levels and capable of creating an army of slaves (it is entirely possible that any zombies they make will obey them rather than the Director). By this virtue, the survivors themselves might be a new type of special infected, and the Director only threw the horde at them so the survivors would be percieved as heroes when they return to society. Infectable people will want to shake the Carriers' hands, eat dinner with them, get a speech from them, anything that involves contact with the Carriers, it is likely to happen. Why not just kill them, you say? Because they are super-intelligent (enough to pick up a complex weapon system and use it like a seasoned proffesional), have a sixth sense (the auras), and are super-strong, -fast, and durable. They don't even attack alone and haphazardly like other special infected, even "more dangerous" ones like Tanks-- they are methodical, and always work in a (possibly psychically linked) pack of four. And it's not even like you can interrogate them for information-- they quite likely don't know that they are anything but lucky enough to be immune. Thus, if you attack them, they will fight with the aforementioned prowess in addition to a cornered-animal-like despiration. The Director went from Evil Overlord to the Anthropomorphic Personification of Paranoia Fuel itself--If you expose them for what they are, you are already infected.

Obviously, this won't happen. But would it be awesome?. Then again, maybe not.
 * Hinted at by the original survivors in The Passing, Francis states that he has had "enough of the military", and Louis wonders if they should've told the new survivors something... maybe CEDA's just a front for BlackWatch?

The Left 4 Dead 2 survivors are The A-Team.
Let's see here...
 * Coach is B.A.: Both seem to have similar means of speaking and tone, they're also about the same size albeit in a different way. (B.A. lost some of his hair, became a Coach, adopted the name, and put on weight. He also somehow lost his fear of flying.)
 * I don't know about "lost" so much as "thousands of homicidal zombies forced him to get over".
 * Ellis is Murdock: Both characters may or may not be insane (Ellis has Keith and Murdock has...everything), and have a similar accent.
 * Nick is Hannibal: Nick is a con man, Hannibal tests his team's clients by deceiving them into thinking he's someone else.
 * Rochelle is Face: Face managed a perfect disguise of his opposite gender and skin color, but as a side effect, is trapped in character and in disguise and seemed to forget who he really is.

Left 4 Dead is the precursor to The Zombie Hunters
Both have special versions of the zombies, and the carriers in Left 4 Dead are already similar to the hunters in the webcomic, right? Unless I'm completely missing something, the only logical jump is that the infection has gone to the extreme yet, and tha thumanity has not restricted itself to the ARK island yet.

The Survivors are insane.
Okay, every single person you pick up is a replacement for someone that you know? A perfect replacement, down to sweat stains and eternal hatred of beards? No, that's crazy. Obviously, many stressful days together have broken the Survivors, to the point where, when they lose one of their number and rescue a brand-new Survivor, they force the new one to take over the personality and mannerisms of someone they already lost.

Boomer bile is sweat-activated.
Boomer bile attracts the Horde. But by that logic, it should attract a Horde whenever a Boomer gets shot/blows up -- after all, it's still releasing bile into the air, right? So the bile must be sweat-activated, like skunk musk. It's only when the Boomer vomits onto a sweaty individual (and the Survivors have to be pretty groady after a couple of weeks) that it attracts the Horde. By extension, those bottles of puke you pick up in Left 4 Dead 2 don't just contain bile, they contain activated bile.
 * So how come the horde doesn't attack the jar? Does said jar hold the scent in as if airtight or what?
 * It would have to be air-tight, yes. Also, probably glass rather than plastic.
 * Perhaps the Boomer bile requires both sweat and oxygen to "activate." Therefore, bile jars contain bile and sweat, but no oxygen, and when thrown and shattered, the oxygen "activates" the bile and attracts a Horde.
 * The boomer bile you find is manufactured by CEDA. They may have found a way to change its properties, or it may work differently since it's artificial.
 * It's entirely possible that CEDA was doing in-field experimentation to try to figure out how the Boomer's bile works. They succeeded before they retreated, and ended up leaving behind bottles they didn't get time to use. Also, yes, those "jars" are in fact plastic, airtight specimen bottles. CEDA uses them to collect biohazardous specimens - they have to be airtight to be approved. (Though that raises the question of how they explode on impact. Perhaps the survivors remove the cap just before they throw.)
 * The jars are thick glass (plastic wouldn't be that clear) and shatter on impact (listen when you throw one). While I pointed out it is manufactured, it may still fit the "pre-activated" idea, since you can throw it at an empty area and the zombies will attack that spot, but if a boomer vomits or explodes and doesn't hit anything nothing will happen.

Immune blood is a deadly toxin for Infected.
The Infected never try to bite you. The closest one to do anything related to mouths is the Smoker, and that's just barely. Instead, they try to beat you to death instead of infecting. Perhaps what makes the survivors immune also causes the infection to die out in the body, but killing the host in the process. That explains all the noninfected corpses. As far as this one could tell, none of them had any visible bite marks and probably died from internal bleeding. Maybe a sort of blood spewer would be a better weapon than guns.
 * Not actually. Sometimes, the Survivors remark that they are covered with bitemarks. They do bite you, you just don't see and probably would be incredibly difficult to make it plausible to program this.
 * The Left 4 Dead 2 Hunter will, once every five seconds or so, bite a pounced target on the neck. So, no.
 * If it helps, imagine you getting slowed down by zombies as them grabbing onto you and biting you while you're trying to run.

The Zombies Will Be Stopped... But Then The Combine Will arrive.
Here's my theory on the timeline: Half-Life 1 - 200X, Left 4 Dead - 2009, Left 4 Dead 2 - 2009, 7 Hour War - 2012, Portal - 201X or 202X, Half-Life 2 - 202X, E1 - 202X, E2 - 202X and E3 - 202X
 * I shoot a hole in your theory. Portal has to come before Left 4 Dead 1 and 2 because Still Alive plays in The Parish's jukebox.
 * So what you are saying is upon writing a song to lament to her test subject's escape and her own non-demise she preceeded to have it published to the common public? I find "Easter Egg" more plausible.
 * And I back his/her theory, what was GLaDOS said? "Are you trying to escape? [High-pitched laughter] Things have changed since you last left the building. What's going on outside will make you wish you were back in here. I have an infinite capacity for knowledge, and even I'm not sure what's going on outside. All I know is I'm the only thing standing between us, and them... Well... I was." -- She was talking about the infected!
 * ...or the Combine. Then again, she says she knows everything. She would know about the Combine invasion and the Infection. It's more likely that the facility was built on a large plot of land and two teams of different color schemes are constantly killing each other over it for no clear reason.

The AI director caused the zombie apocalypse.
He's a necromancer type guy, who can control the zombies now. He's out to get you as survivors, obviously. He's also bipolar. Sometimes he feels remorse and gives you useful stuff instead. This transition is heavily modulated by other emotions such as pity and fun.
 * So, does that mean that Expert mode is really just the director remembering to take his medication?
 * Alternatively, the AI Director is GLaDOS. Because this is so what she would do.
 * Alternatively alternatively, the AI Director is a Mad Scientist who engineered the infection and released it on the world. Given that it seems to be biological, it might make more sense than necromancy.

The zombie virus is a mutation of Creutzfeldt-Jakob disease, also known as mad cow disease.

 * Think about it, putting aside the signs in the airport and the piles of cow corpses, the symptoms of mad cow include:
 * Dementia
 * Hallucinations
 * Speech Impairment
 * Jerky movements
 * Balance and coordination dysfunction (This is a big one)
 * Involuntary movements
 * Hate to shoot this one down, but the offical backstory says The Infection is a mutant strain of rabies.
 * Not to be contrary or not accepting of possible theories, but is there a source for that? Because the symptoms of Mad Cow fit just as well as those of Rabies, if not better. It could be that it's Rabies and the government thought it was Mad Cow, hence the signs and cow piles.
 * What official backstory? All we have is the writing on the walls last i checked.
 * It's all there in the manual.
 * Unless you've got the PC version, in which case it's All There On The Website, as there is no manual save a scrap of paper with controls and a CD Key.
 * But, as we all know, that's Par for the Course for a Source Game. Whoa. I Just realized the anagrams all over that last sentence. Weird.
 * Also, Mad Cow doesn't spread through biting. It's a prion disease, and we're pretty sure you can only get it through consuming tainted nerves/brainmatter.
 * Totally impossible, as the previous users have said. The website specifically says it's a mutated strain of rabies. The scary thing is that humans in the late stages of rabies DO behave quite zombielike as their higher brain functions are slowly destroyed.
 * All the cows in the cow piles have the skin around their skulls scraped away, which indicates rabies testing.

What the bloody hell is the Witch?
All the special zombies are obviously zombified versions of existing humans - smokers are smokers, boomers are fat people, hunters are gangsters/rapists, and tanks are athletes/bodybuilders. But what are Witches? Teenage girls who are crying because they didn't get to see the rest of the Twilight movies? And of course, for some reason, only women are capable of undergoing this specific mutation. Perhaps it has something to do with the virus' interaction with the Y chromosome?...
 * My theory goes hand in hand with another theory I had. Perhaps the Witches are Infected Slayers.
 * This troper's friend came up with the theory that the Witch is someone who is Infected, but still aware of who and what they are. She sobs because she hates what she's become, and the presence of the survivors enrages her. When she kills a survivor, she bursts into tears again and flees - because she realises that she has just repeated the cycle she's unable to escape from.
 * A similiar theory is that the Witch has had all her senses enhanced greatly. Sight (flashlights become mini-suns in her eye), gunshots and voices become sonic booms, etc. Hence the attraction to sugar. The smell/taste is sweet and they remember enjoying it in life. On top of this, we should all know what our 5 senses are...Touch is one of them. This means that the rapid growth of razor sharp nails could have caused her immense pain, causing her to not be able to take the sensory overload any longer, ripping apart the nearest thing.
 * This troper agrees with the above troper's friend. cept with the enragin part, heighten sences make it sound like an air raid right next to her ear, great for infected, not for regular people. That is why she is usually bye herself/ off the beaten path/ attacks spec infected when they get close (try it in versus)
 * This troper read somewhere that the Witch is probably a rape victim. Not sure where I read this - probably on this wiki.
 * Alternate Character Interpretation's entry on the game's main page. This troper's the guy that wrote it. It makes an uncomfortable amount of sense- the infected are described as people who've lost their humanity, with only anger left behind. It's not hard to imagine a horde's assault taking on a different form.
 * Another viewpoint would be that the special infected aren't that clear-cut. Human smokers aren't super-tall with hoses for tongues, so special infection can obviously have dramatic physical effects. Who's to say a boomer would have been a fat guy prior to infection? The same non-explanation may be true of witches; they don't have to have been anything. But here's another idea anyway: witches were women at a certain stage in their menstrual cycle.
 * This makes more sense when you onsider Tanks. Tanks have about half a ton of muscle on them, and are over eight feet tall. There's no way the guys were that huge before they were infected.
 * Wouldn't you expect far more witches then?
 * The Hard Rain campaign in Left 4 Dead 2 has a witch every 5 godforsaken feet. They must be attracted to rain.
 * Word of God says that they're attracted to the smell of sugar.
 * Maybe the rain smells like sugar?
 * Or maybe they're, you know, attracted to the SUGAR mill. Just a thought.
 * Obviously the witch is Bad Girl from No More Heroes.
 * Perhaps they're mothers infected by their zombiefied children, or infected mothers who ate their children.
 * OR the witch is the most intelligent Special Infected because it doesn't run on just instinct. Think of it as a zombified Angler Fish. It cries only as a lure to get you to come over, and it jumps up when you get close. It's merely just being sly.
 * The simplest notion is that all of the special infected are simply people whose genetics reacted to the virus in specific ways, causing mutations. Like all of the infected the witch has a bit of a temper, but it's not quite as hair-trigger as the others.
 * Ever notice how the hunter has tape on his arms and legs, at least in the first game? What's up with that? Was he one of the paranoid victims who thought he could save himself from air exposure to the virus, or a Genre Savvy guy who was trying to stop zombie bites?
 * They are parkour practicioneers. The tape keeps their clothes clinging to their limbs, so they wont get cought up on something while bouncing around.
 * The Witch is a former Survivor. It stands to reason that, in the beginning, not all Survivors would've been immune. The Witch turned while on the run from the rest of the zombies, and her True Companions left her -- hence why she's crying, and why she's always in your path; it was the path she was following, not all that long ago. When you startle her, the virus takes over and makes her attack you, but once you're dead she comes to a realization that she has become what she hates.
 * It seems clear that psychological trauma seems to be a large factor in describing who the witch used to be. As mention above, perhaps she was a rape victim, or is still mentally aware of what she has become. I think that she's crying because she's aware that her transformation resulted in her killing a loved one, and now she cries about it.
 * This Troper recently realized what Witches are. His uncle was talking about the Salem Witch Hunt and said that the women who were considered witches were really just menopausal and post-menopausal women, who tend to be anti-social, emotional, and can sometimes become pissed at the drop of a hat. The mutant rabies can't really help the matter and could probably even turn a woman who isn't like that into a emotional psycho who can go from crying to tearing your larnyx out in a few seconds. This still doesn't explain why they flocked into the sugar mill, but it's a start.
 * I don't know, what could possibly draw emotional, hormonal women to sugar? Oh, wait...
 * Witches are rumoured to be pregnant women. You know, mood swings, attraction to sugar.
 * The Witch wearing a Bride's dress in the Passing summons an entire horde of wedding attire zombies. Looking at the tattered rags of the normal witches, it's just possible that they could be a shredded wedding dress. This might also account (as well as the pregnancy or menospause theory), for why there are so many of them. Every woman who turned during her wedding becomes a witch, becoming sad because of the destruction of her special day (which would also have had all her family and friends gathered together to maximise the emotional punch.) Something about the optimism linked to having a wedding kept them from changing completely, and so they retain a semblance of who they are, thus leading to sobbing guilt ridden existence.
 * OR: Grand unifying theory attempt, Witches are pregnant brides who were on anti-depressants to try and keep themselves going through the big day. they turned on their wedding day, the fetus got absorbed into their hands to form claws, but the combination of medication and pregnancy/joy of wedding managed to stop them from turning completely to mindless zombies. Thus when they incap or kill a survivor, they run away because they realise they almost ended someone else's one chance at a normal life, which drives them into paroxyms of guilt.
 * The witch is a young girl. The virus caused, among other things, rapid maturation without some of the psychological effects. Much of her behavior can be chocked up to immaturity.
 * The infection obviously attacks the brain, causing atrophy of higher mental functions and extreme emotional/psychological states. Look at the basic behavior of the infected, ignoring the physical changes and the behavior that results from them. Just about every common zombie, the Charger, and the Tank is permanently on RAAAAAGGGGEEEE mode when they see a survivor (and it is likely that outside-of-gameplay, infighting is fairly common). Hunters and Smokers seem to be on a primitive "predator" mode, being all sneaky sneaky while stalking prey and then exploding in violence when the prey is pounced/ensnared (the Hunter attacks seem to be notably similar to a chimpanzee mauling than a human beating). Jockeys are obviously manic, cackling almost constantly. Perhaps Boomers and Spitters aren't very well developed beyond expelling the contents of their bowls, but they likely have some psychological traits outside of basic gameplay, however they seem to have a compulsive nature to them, as if they can't help doing what they do, yet at the same time don't really want to get caught doing it (ninja boomer explained) - it is notable that they never seem satisfied in their actions against the survivors, not the way the other infected do after a kill (Tank RAGE notwithstanding). Coming to Witches, they are clearly more fearful than outright sad. They are scared shitless, no question about it. Spook one, and they'll teach you why it's called FIGHT or Flight. Their attraction to sugar can be explained in that it's a key ingredient in any comfort food, which humans are instinctively compelled to eat when they're feeling down and out. Loose your job? Goldfish die? Zombies clawing at the door? Ten bucks says you'll spend your time devouring a tub of ice cream. As for WHY people become special infected, I think we're going the wrong direction in thinking the virus even cares who or what anyone was before they were infected. The virus just attacks cells in the brain and other systems, nothing more. In my opinion, Witches and Tanks are not as One-Gender Race as they appear, but instead are counterparts to each other. Think of it this way: male brains first response to fear is to attempt to eliminate its source directly. Alien attack? How can we beat em? Female brains react to fear by trying to find an exit of the situation. It isn't cowardly, it's just an indirect way of removing fear. If the source doesn't go away or follows them, Mama Bear ensues. The opposite happens in males, as if they can't defeat something, they opt for a "tactical withdrawal," though this isn't the case in Tanks due to Suicidal Overconfidence. Tanks and Witches are basically the same strain of virus, it's just testosterone causes a most of the changes. Ever notice how friggin' strong the Witch is? Yeah, she's a female Tank.
 * Getting slightly off topic, but this troper felt obligated to talk about the boomer's and spitter's psychological condition: The boomers are people depressed about their obesity, and the spitters are bulimic women.
 * A theory expanding upon some of the former: The Witch is an infected who's mutation was mostly of the brain. Sure, there are claws and such but on the surface, the Witch still looks far more human than the Boomer or the Tank or whatever. Their brain, however, was transformed in a way similar to the way the bodies of the other special infected were transformed: it was basically super-duper charged, to a destructive degree. This explains Witches retaining their human consciousness: even the mental blur caused by the virus can't completely negate their new, twisted genius (merely bringing them down to normal human level). They are capable of feeling fear, sadness. Who knows? Maybe they can even still talk. Unfortunately, their new super-brains also made all of their senses incredibly... well, sensitive, which is why they keep to the silent and the dark. Light and noise hurts them. Their brains also provide them with vastly enhanced reflexes, explaining their deadliness.
 * Both the Witch and Jockey suffered from Bipolar Disorder. The Witch was in the depression phase of the disease while the infection hit, and the jockey was in the manic phase. It would explain the behaviors of each infected, as well as the similarity in coloring of their claws/hands. same symptoms.
 * At least some witches have to be adult women--graffiti in The Sacrifice mentions that a survivor's mother "killed Dad when he tried to stop her from crying. She's one of them."
 * Go on Google and look for pictures of Anorexic women. Minus the claws, they are the exact image of a witch. Similarly, many anorexics suffer emotional disorders, constantly feeling awful about their bodies, wanting to be left alone to cry. This may also explain that boat full of ex-model witches in the Comic. As for the Sugar, they may like the smell, but notice they are never seen eating it.
 * This troper's theory is that witches are a form of Tragic Monster- their still partially sentient, and aware of what they've become. Their crying because they still can't control it though, and they don't want to hurt you, but they have no choice...
 * Another theory: The Witches are actual witches, i.e. practitioners of witchcraft. When they were infected, since no medicine could help, they attempted to use spells and/or alchemy to cure themselves. Unfortunately, the cures only half-worked, and left them with the appearance and mannerisms of an Infected, but they initially retained sanity and higher thought processes. But after living through two weeks of painful infection and isolation (any other Survivors they try to join would shoot them on sight), they're reduced to crying and severe depression (possibly including self-harm, considering the claw marks on their bodies). When they're suddenly disturbed, they panic and attack the interloper. This doesn't explain why they're all female, though.

The survivors are psychically linked to each other and the infected.
This explains the auras and health bars at the bottom of the screen. Being linked to the horde explains the music, as well.
 * Gameplay and Story Segregation. Ahem.
 * How about being "immune" to the infection (or rather, the rather nasty symptoms) instead causes other more useful symptoms like the ability to see other immune people?
 * The auras around important inanimate objects could be the result of an enhanced survival instinct, which causes the survivors' brains to intuitively recognize and "highlight" items which aid their survival(weapons, medical supplies, and ways with which to clear obstacles).
 * And the ability to see each other through walls is probably a representation of senses we don't get in video games (4 directional sound, feeling vibrations, Francis's body odor, etc)
 * Left 4 Dead 2's realism mode disables those HUD interfaces, making the game a whole lot harder. It shows why Valve gave you them in the first place: The game is horrendously hard without them.

The survivors ARE infected.
In fact, Everyone left on the planet is. Take the Church Crescendo event in Death Toll, the infection was supposed to have wiped out the population, but the survivors who were immune. Yet two weeks later, the guy snapped and still turned into an Special Infected. His immunity only lasted so long before it eventually gave out. In fact, given that he didn't just turn into a regular infected but into a Special Infected, might mean that those who are "immune" aren't, but the infection for the special infected just happens to take longer, which makes those people feel that they are "immune".
 * If you go by the theory that the infection is only spread by body fluids, then it's possible for someone non-immune to avoid being infected if they're careful and live in a properly fortified shelter like the Church Guy. Furthermore, listening to his sound files seems to imply that he only got infected by allowing an infected person(who claimed to be immune) into his shelter and getting bitten, around an hour before the survivors reach him.
 * It's not so much "imply" as "outright state" in his deranged mumblings. "I can't believe he bit me," "You said you were immune," "It's been an hour, I must be immune," etc.

Moreover, the survivors you control have special abilities other humans don't. You can sense and see through walls to see if your partners are in danger. Bullets have lessened their deadliness (depending on the level of difficulty) and on the lighter ones, being shot does little more than to annoy you. Which implies that the infection has taken hold and will start to change the survivors sooner than later.

And for your pleasure, What survivor will turn into what Special Infected:

Francis: The Tank. No one else loves the fight as much as he does and he just always wants to get into it.

Louis: The Hunter. He seems the most apt to run around.

Zoey: the Witch... totally no need to go into more detail here. Hell, she's even wearing red.

Bill: The Smoker. One of the slowest infected with a hobble. And... he smokes.
 * debunked though, pretty sure that's there just to help you. IN realsm mode (Left 4 Dead 2) there is no wall vision for survivors.
 * Awwww...Who will play the Boomer?
 * Church guy, of course!
 * Swooping in with some supporting evidence: "They're changing." If the special infected are just now beginning to appear, as the game opening suggests, then all the more reason the "survivors" haven't mutated yet. The special strains simply have a longer incubation/transformation period.
 * The graffiti on the wall in one of the safe houses mentions that different people mutate at different rates, with some taking days and some less than five minutes. This gives a little more credit to the idea that the survivors could be infected and not know about it.
 * This guy has a different take on it.
 * It's hinted in Left 4 Dead 2 that the survivors are immune, but they have the Infection and spread it to anyone who isn't.
 * Actually, there's strong hints that was just public paranoia and rumormongering, as the public is wont to do in a major crisis. There's a few of them, but the most damning is: If the immunes/carriers are actively spreading the disease just by their presence, why didn't the military just leave them to die on the bridge? Holding a helicopter evac for 10 minutes and stalling a bombing run seems like a lot of effort to go to for four people who turn everyone they meet into zombies.
 * Because only the immune/carriers are able to fight the zombies without risk of turning into one?
 * But think about this. In both Left 4 Dead and Left 4 Dead 2, the respective Survivors are rescued by a helicopter. And in both instances, the pilot becomes Infected, loses control of the plane, and is killed by the Survivors. Now, which is more plausible: that both of these pilots just happened to have been bitten/infected by a common infected prior to rescuing the Survivors, or that the Survivors were in fact the ones who spread the virus to him?
 * The former because both choppers were seen looking for survivors before the survivors you control could contact pilots and it's highly possible the pilots had found people beforehand and took them to safety and one of them could have been infected.
 * To add to that, the No Mercy pilot says he had "an... incident" over the radio, and I believe it's been said in various interviews and such that he was bitten by an infected. That could've happened to the Left 4 Dead 2 guy too.
 * The characters in both games are immune. All carriers are immune, but not all immune are carriers. CEDA didn't realize that there could actually be carriers and took in any non-infected. People who had no contact wouldn't know if they were immune or not. When thrown in groups with carriers, the carriers infected some of the survivors resulting in the epidemic. The survivors are completely immune, meaning the disease doesn't exist in them at all.
 * Well now, The Sacrifice comic/DLC pretty much confirms that at the very least, the Survivors from Left 4 Dead 1 were carriers, being set after the end of Blood Harvest, and I won't spoil any more, for the people who want to play/read it.

The Virus is sentient and evolving.
It also has a Hive Mind. Wonder where the Spitter, Jockey, and Charger came from between Left 4 Dead and the sequel? The virus created them to counter the strategies the original Survivors used.

The Survivors are immune because they're all addicted to pain pills
Think about it. We see them gulping down entire bottles of them at a time, without water or anything. Because they can do this, they're obviously pro pill-takers, or to use a different phrase, addicts. The beginning of the intro movie states that the zombification is caused by an infection, so the conclusion that therefore, there's something in the pills that has built up in their systems and has prevented or granted an immunity to the infection to the Survivors doesn't seem that far off.
 * Except it's fairly clear that the pills are bottles of Ibuprofin, a liquid-gel, aspirin-derived analgesic and from the rattling sound they make, there's not much left. The survivors are likely guzzling down the last bit.

The infection is related to the serum from the Doom movie.
99% people turn into mindless zombies. The rest can shrug off their attacks, become incredibly resistant to damage (on lower difficulties), can heal up pretty damn well, scarf down tons of pain pills, have an instinctive danger sense (the soundtrack) and can see each other through walls, as well as detect items of interest. Sounds like superpowers to me.

Each campaign in Left 4 Dead 2 will take place in a different city, and possibly have different characters for each "movie"
Just because Rule of Cool demands that the game has a chance of sending you to Las Vegas or Times Square. The different characters would allow for an encore performance by Zoey and the gang.
 * Confirmed. It's in New Orleans (at least for one Chapter).
 * To clarify, the game starts in Savannah, Georgia, and the survivors will end up in New Orleans.
 * Pretty much confirmed, yes. The survivors in Left 4 Dead 2 travel quite a bit thanks to obtaining a vehicle early on, and eventually make it to New Orleans via boat, where they're evaced via military helicopter.

The infection is a strain of Blacklight.
Okay, in Prototype Blacklight infection typically results in normal zombies, easy to kill. But it can also bring about far stronger monsters, like the enhanced infected, hunters (not the same as the Left 4 Dead ones) or hydras. Others, exposed to a different strain are just eight feet tall, superstrong, and able to follow military orders just as well as any soldier. The more "wild" Blacklight mutants (and zombies) are under the control of Elizabeth Greene, whom may be the director. And looking at Alex Mercer, he's like several of the boss infected's powers all rolled into one package.
 * What, the T-virus not good enough for you? At least speculate on games that came out before Left 4 Dead.
 * I'm pretty sure that most of the infected in Prototype were infected with Redlight. It would make more sense if the survivors were infected with Blacklight, given that they still appear human, but can shrug off bullets, see through walls and decapitate zombies with a cricket bat.
 * Sorry, my mistake. Then again, if the survivors were infected with Blacklight, then they obviously don't know the full extent of their capabilities, or else the game would be over in about 15 minutes.
 * Alex had to eat a lot of people to unlock most of his abilities. It's not something most people would try doing in a zombie outbreak.
 * Even so, Alex ran at superhuman speed, could leap ten feet straight up, and run on walls from the get go.

The "Director" is actually a powerful psychic.
And their abilities are increasing. Before they could only control the undead, now they can control and manipulate inanimate objects (they aren't too different from human corpses, after all). The mutations are their experiments on twisting the body into different shapes. Why does this matter? In the final game, you will meet the Director, and you will kill them. It won't be easy, a shield made of whatever they can get their hands on and hordes of zombies and objects attacking.

The Director is actually producing the movies seen on the posters.
And the survivors? Method actors taking place in a bold new experiment. Their world is Sufficiently Advanced enough to have cloning technology, and the Left 4 Dead series is a bold new frontier: 'real-life zombie horror' with unscripted combat, terror, and death.
 * If the survivors are killed in all but the last level, they're just revived and 'rescued' later on. Death's more permanent in the finale to up the drama -- they should've read their contracts better. (They probably do get brought back anyway; they just get told otherwise for better reactions.)
 * Of course, they are geniunely terrified -- even knowing in the back of your head you're just going to be brought back, being torn apart isn't precisely pleasant.
 * The zombies are probably all low-quality clones -- an unpleasant side-effort of not having all the bugs worked out of the technology at first. Some corrupt corporation just decided that hey, lemons and lemonade, right?
 * It follows that the special infected are also genetically engineered clones, and that they're all dressed identically because you get a bulk discount if you order a thousand pairs of Boomer-sized sweatpants.

But hey, it could be worse... Just consider what the Reality Shows must be like in this world.
 * Dead Rising 2 and TIR, anyone?
 * What's TIR?

The whole thing is a quadrilogy of movies.
Similar to the above theory, The survivors aren't survivors, they're just acting in a movie, the director decided to buy quite a bit of land to make a movie, and got every civilian to somehow act out the role of infected, the sequel is made by a different director, who decided his vision was better.

The AI Director is putting everyone through a broken Matrix.
The 4 human characters are in The Matrix, but it's broken and zombified 99.99% of it's occupants, has given up trying to get things back to normal, and is just widdling away eternity watching these 4 people try to get out of a situation they have less than no chance of succeeding in, even going so far as to bring them back to life when they fail and adjusting the playing field as they go.
 * Then explain the lack of the Deja Vu created by a system change.

The Last Stand is canon.
The northeast safe zone gets overun at some point after Blood Harvest. The survivors make for the coast. In accordance with Left 4 number 2s plot,  They crank up the lighthouse in an attempt to signal one of these boats. Rocks Fall, Everyone Dies.
 * Can't be. Valve recently announced a DLC campaign where you meet the original survivors as the new survivors.
 * Who says that The Passing (which is set between Dead Center and Dark Carnival in the Left 4 Dead 2 timeline) isn't set before The Last Stand? ("Hey, guys, thanks for your help! Now let's get to that lighthouse...")
 * The Last Stand is a 'What if' scenario, showing what would happen if they took a wrong turn.
 * Valve is probably going to make DLC of the Left 4 Dead 1 crew making it to the Lighthouse and surviving. The Last Stand lighthouse is probably just the prototype for the level.
 * If they can contact Bill's voice actor, who likely isn't happy about The Passing...
 * Valve is probably going to make DLC of the Left 4 Dead 1 crew making it to the Lighthouse and surviving. The Last Stand lighthouse is probably just the prototype for the level.
 * If they can contact Bill's voice actor, who likely isn't happy about The Passing...

The Spitter and The Jockey are a married couple that turned at the same time.
Both of them are missing their lips, probably from kissing as the acidic saliva of the Spitter burned them away. They were also designed to work together, the Jockey moving Survivors into the Spitter's acid pools.
 * On that note...

The Infected are deliberate stereotypes.
Morbidly obese freaks stuffed to bursting... with infectious, horde-attracting bile. Shady-looking street hoods... that can leap a dozen yards to hack people apart with their razor-sharp talons. Weepy, sugar-scenting depressed women with hairpin triggers... and bone-carving talons. Bulky idiot Alpha Males... that can toss cars and chunks of concrete. Smokers who spread their disease to those around them... aggressively, and with plague-dripping attack tongues. The new Infected are pretty obvious parodies of stereotypical redneck traits with horrible mutant twists. Why is this? Because...
 * But what of the sequel? The charger is possibly a sterotypical... what? perhaps he was a football player of some sort, he can barely speak and what he says can't really be classed as speach, it sounds more like some kind of dumb aninmal imitating human speach. The jockey is possibly some kind of mental patiant, Laughing Mad and possibly one of the most disturbing infected (he climbs on you and freakin humps you to death!) but what the hell is the spitter?
 * That can be spotted through the spitter's clothing. From the looks of it, she once was a normal preppy teenager. With bulimia.
 * The charger was clearly... err.. not master of his domain.
 * No no no, think about it. The Charger wears nothing but overalls, runs into stuff without the ability to stop, no matter where he's going and can't immitate human speech? Clearly he's a stereotypical SOUTHERNER!f

The Infection is supernatural and has a black and immature sense of humor.
The "Green Flu" has a mundane component, mutated rabies. But that's not the real source of the infection. A malign supernatural intelligence is carried through the disease, starting off weak and growing exponentially stronger with each new victim, culminating in the creation of the Special Infected and the Horde. Why are the Special Infected such grotesque parodies of human stereotypes? Because the Infection thinks it's funny. The Infection designed its own traits deliberately to exploit human weaknesses and fears to better destroy them. While the supernatural component gives the disease its hideous strength, it also provides a glaring weakness--The Power of Friendship is not merely helpful for surviving the apocalypse, but necessary to shore up against the selfish impulses that enable the Infection to steamroll individuals and those who are merely disciplined without real attachment to those they're protecting.

A possible Infected hierarchy, assuming the Infection is intelligent.
Much like Valve, the infection's mutations are deliberately tactical. The emphasis: spread the disease and prevent predictability! Humans can efficiently resist zombies when they fall into predictable routine, so "early" SI exist to spread the disease as potently as possible and "advanced" SI scatter efforts to resist zombies efficiently.
 * The Horde: Common people infected and turned to ravenous zombies. Most important as shock troops--the infection is spread through bodily fluids, so their insane rush to melee ensures their victims will be, at least, splattered with blood and other substances, likely over wounds inflicted by the Horde. Alive or dead, their victims will eventually turn into more Infected, feeding the Horde. This is handy but inefficient and predictable (and easily dealt with via ranged weaponry), so that's why we have the Special Infected.
 * Boomers: The first strain of special infected to develop, and the most important for spreading the illness. Boomer bile directs the horde to sensitive targets and is, of course, a hellaciously potent disease vector (partly why the infection is called Green Flu). Boomers are high priority for CEDA because of how casually they can infect groups of people and spread chaos.
 * Spitters: A refinement of the Boomer mutation, allowing direct damage and acts of sabotage in addition to rapid and potent spread of disease. Halfway between the Boomer (rapid disease spreading) and the Witch (area denial).
 * Hunters: Secondary special infected. Dead victims turn quicker than live ones, generally (depending on immune system strength, some humans last for a long while and others not so much), so Hunters exist to help speed the process of infection by hurtling human victims into the Great Egress. Because they're also fantastic distractions they're among the earliest SI to develop.
 * Chargers: Midway between a Hunter and a Tank. They're there to speed along the dying process and thus bolster the ranks of Infected. These generally show up at the same time as Hunters; the original four survivors were just lucky enough to not encounter them.
 * Jockeys: The ultimate undead wingman, designed to assist others in their deeds. While technically the least dangerous in terms of direct threat to health, these cackling little bastards are necessary for keeping the element of surprise on the infection's side.
 * Smokers: Necessary for creating the ultimate infected. They smolder because of the potency of the infection roiling in their disgusting tumors. Their "tongues" are coated with the "kingmaker" virus, which turns male victims into Tanks and females into Witches.
 * Tanks and Witches: Zero infectious ability, maximal destructive ability. These are created specifically to deal with immune survivors and the heavily-armed.
 * Except that all the infected are still quite alive. You shoot off a leg, they die. Chop off both arms and they face plant. You pump enough bullets into a tank to shred all vital organs keeping the monstrosity alive.
 * I believe, although I may be wrong you can feel free to correct me (prefferably without a degrading hyperlink to You Fail Biology Forever) but I believe cell division is speed up by injuries--injuries most of the special infected cause (they could also be designed to aid the horde-- most scenarios with Pinning SI's invovle them holding you down while the horde brutalizes you, again causing injuries) and the resulting rapid cell division means more and more cells are impregnated with Green Flu prophages, speeding up the infection. The Tank and Witche's tendancy to kill rather than maim does fit the bill of an emergency killing tool when dealing with non-infectable targets, and the smoker does tend to drag you away from help rather than try to turn you right there, most likely so you can develop into one of the aforementioned Area Denial/ Heavy Assassination "monarchs". This fits in with another WMG of the Tank and Witch being of the same strain, and is, in my opinion, pure Fridge Brilliance.
 * I always interpeted Boomers as the leaders/generals. Mostly because I always just wait for them and attack when they do on Versus. Probably why I prefer playing as them, now that I think about it.

The church guy is a relative of Team Fortress 2's Scout.
The Scout is said to be the youngest of eight brothers. The church guy is one of them, which explains the similar voice and personality.
 * Considering that TF2 takes place in 1968, Ellis is probably more likely to be his grandson or something of the kind.
 * Both could be possible! Both could have Scout as their grandfather.

The playable parts of the game are Youtube Poops of the films they portray.
If the campaigns are actually films, then all of the shooting the players do are a rapid-fire compilation of the most violent bits of these movies by a Youtube user by the name of "left4dead". The bits when players grief each other are Hilarious Outtakes.
 * That, sadly, makes sense. But then, shouldn't it be set to "Yakety Sax"?
 * Wait, never mind.

Tanks and witches have the same strain of infection.
They both can take rediculous amounts of damage, and have gargantuan killing power. The rest is the result of the virus interacting with the infectee's hormones.
 * Expanding on this, perhaps the Tank/Witch strain reacts with the sex hormones specifically- high testosterone results in increased muscle/bone mass, aggression and impotence, and high estrogen levels can result in dementia, exaggerated mood swings, and irregular menstruation.
 * Meaning men and women get different sides of the coin. Ironically enough, in a straight up fight, the Witch would win due to her high damage output and inability to be knocked back.
 * The Witch cannot be kncoked back by Humans, I'm pretty sure a Tank is stronger than a normal Human. There's no way of knowing if her claws would work as well on his natural armor as it does on fragile Human flesh.
 * This wouldn't happen to be the same fragile human flesh that the Tank has to punch up to four times to incapacitate, vs. the Witch's one claw swipe.

The virus is a Combine bioweapon.
Shortly before the events of Left 4 Dead Gordon Freeman pushes that fateful cart in Black Mesa. The ensuing resonance cascade opens portals between Xen and Earth, where the Combine stand at the ready with a bioengineered virus. They release the virus into major population centers all over the world, where it turns the bulk of humanity into mindless killers. The resulting chaos leaves the planet primed for invasion, and thus the Seven Hour War...and thus the severe depopulation Gordon encounters when he arrives in City 17.

Hey, we all know Portal is part of the Half-Life continuity; why not Left 4 Dead?
 * A valid argument, but even with all the horrible things happening in the series, you'd think they'd have at least mentioned a little thing like a Zombie Apocalypse in Half-Life 2.
 * It would take a bit of retconning, but the rest of the cast have given Gordon so little specific information about what happened in the months and years after Black Mesa Valve could probably still carry it off.
 * I doubt they could've cleaned up such a volatile virus so throroughly. After all, they haven't even cleaned Ravenholm out. Though this does make sense for something that could happen after Half-Life. Following Episode 2, Gordon Freeman and the other rebels beat back the Combine to a draw. In a final act of desperation, the Combine unleash a virus that is far more contagious and deadly than any variation of headcrab.
 * That's where The Cremator would have come in.
 * Then again, City 17 is in Eastern Europe and Left 4 Dead takes place in the US. So maybe the Combine used headcrabs for Europe and The Virus for America.
 * Here's my theory (which relies on the assumption that Half-Life 2 takes place in Britain or at least Europe): I think that the Combine had really only taken control of a few different governments and militaries. The US was not one of those governments, so this means that the US was still largely untouched after the Combine takeover. So, the US declared war on the Combine to help their British allies. The Combine at first tried to attack the US with headcrabs, but the headcrabs were easy enough to combat, and the US easily managed to avoid their zombifying effects. So, the Combine managed to develop a virus codenamed "The Green Flu" that turns most humans who come in contact with it into homicidal, zombie like beings. Because it was harder to spot and combat than a simple headcrab to the face, and because the US never expected it, the Combine successfully managed to infect several of the US states. The Combine decided against using the Green Flu on the people in their own country, since the Combine had a bigger hold on it and didn't need to use something so destructive to combat people in their own country. This explains how Left 4 Dead and Half-Life 2 can exist in the same universe and time period despite the fact that Half-Life 2 had the Combine as a replacement for the military, while Left 4 Dead 2 had a very realistic type of military
 * As I see it by the time of Half-Life 2 the virus would've long since run its course, leaving only immunes to continue the human race. Assuming the first Half-Life and Left 4 Dead take place right around the same time, the epidemic would have 20 years to peter out. No infected are going to last 20 years in that condition, after all.

Ellis is the Southern cousin of Scout from Team Fortress 2
"Kill all sons of bitches."
 * Team Fortress 2 takes place in 1968. That would make Ellis much older than he is.
 * Alternatively, shortly before settling down to start a family in Onett Scout had a tryst in Georgia that resulted in Ellis being born. Which would make Ellis and Ness half-brothers.
 * If not Cousins, Ellis could Be Scout's Grandson.

The Survivors in Left 4 dead 2 all secretly fear and distrust each other
Some of the "Dead ally" voice clips in Left 4 dead 2 suggest that the survivors don't care for each other too much,cracking jokes at the others demise, hinting that they are only working together from pure fear of the infected. Compare games: "Zoey: * Upon Francis' death* Francis? NOOOOOOO! Rochelle: * Upon Coach's death* "I wonder if Coach was his first name, or his last name?""


 * It's confirmed that the survivors in Left 4 Dead 2 aren't too comfortable together. Over the course of the game they get closer.
 * In fact, they just met each other at the beginning of the first campaign, and haven't given their names until midway through.
 * The survivor's responses to each other dying change over time in the game. In the later campaigns, they're very heartfelt. It matches with what Valve did overall for the campaigns - in the earlier campaigns, the survivors haven't yet named the special infected, instead referring to them as "long-necked bastards", "jumper!" and "BIG ONE!". As things progress they develop names for and even tactics for dealing with them. It really gives you a sense of them learning to work better as a team as you go on and fight together more, just like you and your friends are doing.
 * Ellis appears to be the only one who trusts his new Comrades from the get-go. Coach warms up quickly, then Rochelle, and finally, after much pressure, so does Nick.
 * I've always been one to say the mocking was a coping mechanism, Rochelle and Nick take a while to warm up to the rest of the group but also are by nature not as inclined to display emotional pain. Note how the aforementioned Rochelle line for Coach changes from "Do you think Coach was his first or his last name?" that seems to be more trying to lighten the mood of a death in the party rather than disrespect. But then later on her response becomes a very quiet, regretful-sounding "Goodbye Coach...". Nick is much the same, he's emotionally cold but he genuinely regrets when even Ellis, the one he constantly targets for teasing, dies.

The Infected know the Survivors are immune.
The Infected are basically human viruses: never eating, sleeping, or thinking, only infecting. But then come the Survivors, who are immune to the virus and thus are an obstacle for them to infect more and more people. So they don't bite (knowing that trying to infect them is pointless), and instead just try and kill them so their immunity genes don't pass on to the next generation and hinder their future infection rates.
 * Not exactly. Once, this troper was playing a single player campaign with Zoey and was badly hurt (no medkits, just get up), and she said "I have bitemarks everyewhere". They do bite, we just don't see they doing it.

The infection all started with high fructose corn syrup.
Note: This isn't mine. This is off of a guy on the Steam forums. Let's say the infectious agent for the Zombie Plague is not rabies but in actuality a prion as that found in here.

An all this trouble could have started over 10 years ago. A chemical company tests a new pesticide on corn. It passes FDA and EPA regulations. In 1992 it goes into limited use in the States then mainstream in 1998. After time the pesticide cuases celuar damage to the corn that is undetectable through general testing. Most quality testing on food products is for specific bacterial contamination, and the pesticide would be specically tested. But unless you knew exactly what you were looking for a Mass-Spec would be the only means of detecting this pesticide/corn contamination.)

Note the infectious agent is not from the cows, but from the chemical contaminated corn used in the food industry as--[gasp]--High Fructose Corn syrup. You cannot eat anything out of a package without some exposure to HFCS.

This Prion marinates in America's brain for nearly ten years before--resulting in the changes that brings on the gray skin, reflective eyes, and generally anti-social behavior.



Thus OMG zombies are caused by a virus-- We need to treat this as an infection. Call the military. Shoot people who seem to be infected. And remember one misstep and everyone will die. This explains the military's first response: kill first- ask questions later.

Then after some investigation, the truth comes out. This 'infection' has been with us for almost ten years. Everyone in America has the problem. There is no true distinction between â€˜carriers' and those who are immune. No Hasmat suit will protect you, no amount of isolation will save you, and no gas-mask will keep you from turning.

Fate has already flipped your coin-- Heads you're safe, tails you're a zombie.


 * Impossible, sadly, as the Left 4 Dead website specifically states that the disease is a mutated strain of rabies. It's not mad cow disease.

The Infection was created by the Beef Industry to boost sales.
They engineered a strain of rabies to produce uncontrollable cravings for burgers and other beef products. Things just got a little out of hand. This explains why all the cows have their faces eaten off. The infected are eating beef. It also explains why turning on a Burger Tank sign atracts a horde. They think its the only burger place still active and are hungry for some tank burgers.

Keith is Chicago Ted.
After all, he could survive third degree burns on over 90% of his body, third degree burns on over 95% of his existing third degree burns, drowning in the tunnel of love, and military bombings, so there's no doubting that he could easily put these 'skills' to use in killing zombies.
 * Oh dear god you're RIGHT.
 * Gentlemen, I propose a toast to the OP.
 * And Keith's been run over by a lawnmower.
 * Dear god, it makes so much sense...
 * But then, how'd he get to Riverside?
 * On a jetpack made entirely out of zombie corpses.
 * Keith is Chicago Ted. And Chicago Ted is...Tallahassee.
 * Also makes sense. Tallahassee is strongly implied to be a Southerner (so from Ellis's neck of the woods), dialogue from Left 4 Dead 2 states that he's been to Hollywood, he's referenced again in "Dark Carnival" (remember where the final shootout took place in Zombieland?)--and, of course, he's a Crazy Awesome Badass.

Left 4 Dead and Left 4 Dead 2 ends on a Downer Ending.
The military have begun to take over the zombie infestation, and they know that asymptomatic carriers (those who have the Infection but show no symptoms) exist. Grafitti in The Parish states that carriers are being wiped out. Clearly, the Survivors are immune carriers, and the military kills them in order to stop the Infection from spreading. And who picks up the Survivors at the end of every last campaign? The army.
 * The guy on the walkie-talkie also tells someone else in the copter to prepare the hold for carriers. Downer Ending indeed, although it's unclear why they'd bother picking up the survivors.
 * Perhaps to test on them to see why they're asymptomatic and learn more about the virus? Frankly, that's a worse Downer Ending - get used as a guenia pig and then killed.
 * Alt: It's not a Downer Ending - the Military is only saying to prep for carriers because, well, to quote Church Guy - "Better Safe than Sorry". The Survivors are genuinely immune. (This would also fit with Valve's general approach on not stealing the thunder from players)
 * The timing of The Passing would seem to make this unlikely- if (as is currently thought) it's set after Blood Harvest and before The Parish, then the teams meet after Team 1 have already encountered the military. If the army really were rounding up and killing survivors (meaning Team 1 obviously escaped a military base after the end of Left 4 Dead 1), or even sending carriers back into infected areas to clear out the horde, it's very unlikely that Team 1 wouldn't have told Team 2 all about this. Since none of this comes up when Team 2 are preparing to be rescued by the army in Left 4 Dead 2, chances are Team 2 haven't been warned about the military, so Team 1's meeting with them went smoothly. Or the Blood Harvest APC crashed before the survivors reached a military base and the driver died before revealed what was in store for them, I guess.
 * Team 1 apparently didn't make it to the military outpost for whatever reason. If they were about to be killed by the military it's not like the APC driver would tell them anyway.
 * Jossed. The comic book that came with the latest DLC for Left 4 Dead 2 ("The Sacrifice") indicates that they were held prisoner at a military base for an unspecified length of time. Matter of fact, the only reason Bill was still alive (up until "The Passing", anyway) is that the military has determined that immunity is passed down from the father's side, and they assumed that Bill was Zoey's father. However, this leads to an interesting theory about the gene for immunity.

If the gene for immunity is passed down on the "X" chromosome and is recessive, then Zoey and Rochelle would have to inherit TWO copies of the immunity gene; otherwise, the dominant non-immunity gene would override the recessive immunity gene, and they would've "turned". We know from the comic that Zoey's mother was NOT resistant, so she must have had one copy of the recessive immunity gene, which she passed on to Zoey. This also explains why there are more male than female survivors; a male survivor would only need one copy of the gene for immunity (he would not need one on his Y chromosome), while a female would need two copies - one on each X chromosome. If you go back to high school biology class, you might remember the Punnett squares for simple gene dominance (blue eyes vs. brown eyes, blonde hair vs. brown hair, etc). As it turns out, drawing Punnett squares for the immunity gene reveals that there would be about three or four times as many male survivors than female survivors... and hey, what do you know? Six playable males, two playable females (a 3:1 ratio)! This supports my theory, at least in principle.

This leads to a theory about the Special Infected. If the gene for immunity to the virus is sex-linked, what if the mutation into a Special Infected is also linked to defective sex chromosomes? For example, having only a single X chromosome and no Y chromosome, which is the most common cause of Turner's Syndrome in women? Or having extra sex chromosomes (XXX or XYY, for example)? That would also explain the comparative rarity of the Special Infected. Using my previous example, Turner's Syndrome occurs in only 0.0004% (one in 2500) women. Not every sex-linked disorder has as many health risks as Turner's (and in fact may go completely unnoticed... until you get infected with the Green Flu and start to mutate! BUAHAHAHA!)

The zombies are psychic.
How else would they be able to differentiate between a Boomer that explodes on the ground or on a player?
 * Hit Ctrl-F and type "Boomer Bile is sweat-activated".
 * Doesn't work with Heavy Rain. The survivors' sweat would be washed off by the second half of the campaign, and yet Boomer bile still works on them. Moreover, the Witch somehow knows who shot her, even if everyone was standing behind her at the time. And the Jockey would have to have mind powers to influence where a survivor was running by jumping on their head.
 * Boomer bile is based off pheromones. As for the Witch, hyper sensitive tactile sense - just like you can tell if you get punched in the stomach or the kidneys, since she can take multiple gun shots, she can figure out a general idea of where to look. The Jockey just covers someones' eyes and leans, forcing his weight to make the survivor stumble.
 * Pheromones that are only emitted after the Boomer has exploded on the condition that someone was standing near enough, huh? And it would be pretty tricky to figure out which one of four people threw a grenade at you from behind while your face was covered if they were all in the same place.
 * The pheromones might react with oils in human skin. The oils are a lot harder to just wash off than sweat - this is why you use soap when you bathe/shower. It's not unfeasible for Boomer Bile to react with those, which both produces a pungent smell and bonds to the oils, quickly rinsing them off and becoming ineffective again. As far as the Witch? I'm willing to go with either 'she's psychic, thus the glowing eyes', or 'she has incredibly good hearing, smell, and short-term memory, which explains how she can pinpoint exactly who threw, fired or swung whatever hit her, whirl around and beeline for them with unerring reflexes and find them no matter how far ahead in the level they go'.
 * The survivors aren't exactly militarily trained in noise and light discipline, nor in stealth. The infected can hear them very easily, and it doesn't take them long to realize that all that gunfire and screaming is probably not one of them.
 * The infected aren't attracted to sound... they're attracted to high-pitched sound, like the beeping of the smoke detector attached to the pipe bomb, not the bang of gunfire. The comment about light discipline makes sense, though.

Keith is real, but he made up all those stories to trick Ellis.
Ellis seems pretty gullible and Keith could have easily tricked him into believing his outlandish tales. Some of the less crazy tales (like him and Ellis filling up water balloons with their own piss) could very well be true, though. "Ellis: Did I ever tell ya about the time my buddy Keith and I were on the top of a burnin' building, and we had to fight our way down like five floors 'a zombie, and... Wait a second, I guess that was you guys. Aw shit man, I can't wait to tell Keith about that one."
 * Not so; Ellis directly recalls BEING with Keith at the time of the event(s) stating that: "We tried to play bumper-cars with some riding mowers we scrounged up."
 * Alternatively, Keith is real, but the stories are an invention of Ellis's immagination.

Keith is Kenny.
It explains his bad luck and the fact that he survived all the events that Ellis tells the others.

The Spitters used to be bulimics.
I'm utterly shocked that so many people are confused by the Spitters. Left 4 Dead 2's new special infected are all based more-or-less on highschool stereotypes. Chargers are the dudes on the football team, Jockeys are those annoying bastards you just can't get off your back, and the Spitters are those vapid, appearance-obsessed chicks who eat lunch, but then go to the bathroom and vomit it up as soon as they can.
 * Bulimia is a disease that can develop out of being obsessed with appearance, as can anorexia, but once the disease is developed these are legitimate problems that the subject cannot break out of without help. They are chemical changes in the brain. They deserve your sympathy. Scorning them is like scorning a clinically depressed person for being "emo". In any case, that was likely the inspiration for them, but I'm not sure bulimia would cause a virus to react differently to a subject.
 * Eh, not exactly failed forever - the above theory regarding neurophysical changes is still rather uncertain, in part due to a lack of rigorous testing, and clinical trials involving various inhibitors, antidepressants, etcetera have produced mixed results. I would also note that Bulimia would not likely result in a different form of infection from an altered Rabies strain (unless it was somehow influenced by the state of the posterior dental surface...) More likely, the stereotype influenced Valve's design, and was not meant to be extended to a meaningful, in-universe element.
 * In any case, you're Completely Missing the Point. This should go with the stereotypes discussion above -- the infected are based on stereotypes, not specific types of infectees.
 * A better theory would be that the Spitters suffered From Acid Reflux disease.
 * A different theory, more like. I reckon that Spitters are people who had their appendices removed (so the virus would be able to seize control of the entire digestive system faster, and convert it into an acid factory)

Rochelle will become a new Director.
After all, she's already called "The Producer". Perhaps, if the earlier WMG about the military taking the survivors in to test why they're immune is on the mark, this will turn out to be part of The Director's Plan: grooming a partner-in-crime-slash-successor. Just who offered her that big break covering the Savannah evac center, anyway...?


 * Makes sense since the director is purely evil, Rochelle can extract revenge on all the people who hated her since day 1 of the game.
 * She may also team up with Zoey, Who as stated in the comic, wanted to be a director. The Current Director may Favor Rochelle over Zoey because Rochelle has more experience.

The special infections are all caused by reactions to drugs.

 * Tanks: Steroids
 * Smokers: Nicotine/Tobacco
 * Boomers: Alchohol
 * Alternatively, Boomers could be addicted to super chemical, McDonalds style fast food.
 * Hunter: Crack/Heroin
 * Spitters: Diet pills* Specifically, pills speeding up the metabolism. The mutation made it so that the stomach acid became much stronger*

Still a little on the fences about Jockey, Charger, and Witch, although maybe the Jockeys are schizos on Medication on anti-depressants. Witches may also be on anti-depressants.
 * I like this theory. Maybe the Witches are caused by excessive hormones instead of drugs?
 * Oh, please. Witches were obviously infected during that special time. It's why they crave sweets.


 * Jockeys, due to their kinda hyper-active tendencies, could be addicted to caffeine pills or energy drinks. Maybe the Charger used some injection drug and his ugeass arm is due to contaminated needles.


 * Jockeys are meth-heads.
 * The chargers come from guys like this one: wrestler.jpg
 * That... is really disturbing.

Zombieland is in the same universe as Left 4 Dead (hey you knew it was coming)

 * Later the infected from Zombieland starts to mutate into the Left 4 Dead "zombies". Some of the "zombies": from ZBL are seen drooling green slime. The same green bile that's seen in Left 4 Dead.
 * Remember the supermarket scene? The fat zombies were immature Boomers. If they got there a short time later Tallahasee would've gotten "Clean Kill". You can't tell me that wouldn't be Zombie Kill of the Week.
 * There was also the zombie clown (which was "Cl0wned").

The survivors and special infected never actually die.
Think about it, the survivors suddenly and mysteriously reappear halfway through the game for no reason, and all of the special infected look the same as all of the other special infected of that type (E.G All hunters look the same, all Boomers look the same (except some are women) all tanks look the same, etc.). It's possible that the game is a fictionalisation or some other handwave, and none of the special infected or survivors actually died in the real version of the story. It also explains why the special infected changed appearance slightly in Left 4 Dead 2, they've just mutated more.
 * So the Smoker took the time to change his shirt in between games? He went from a jacket and t-shirt to just a blue t-shirt.
 * Some of the special infected did change appearance... they introduced a female Boomer, the Smoker has more tumors (and six tongue-like appendages instead of just one, though it only uses the one in its mouth to snare you), the Tank's muscle mass has increased to the point that its lower jaw is basically merged with the chest, the Hunter's arms show increased abrasions around the wrists (possibly due to trying to claw its clothing off, or just calluses from all the jumping around he does), etc. However, this is the result of continued mutation, not a case of "they never actually died".
 * Actually, in realism the only way for survivors to come back alive is through the defib unit. Plus, the rescue closet was explained in an interview that what happens is that the "survivors crawl a certain distance and barricade themselves in it. After they recover they realize they can't get out, and call for help." As for the infected looking the same, Valve didn't want to make a different skin for EVERY SINGLE SPECIAL IN EACH LEVEL. Plus, it's possible for 2 of a certain infected to be on the map at the same time.
 * I always understood it as Survivors being locked in closets by their old team-mates after they got bitten. They eventually realised they were immune but they couldn't get out (despite having guns well equipped to shoot the lock off)

We missed some infected.

 * However, they aren't going to kill us. Or at least not yet. There are 8 special infected, right? My theory is that there are 9. The 9th infected doesn't physically mutate, but their insides twist around and give them special abilities that normal infected don't have, like enhanced senses, more physical endurance, more physical strength, regeneration, enhanced metabolism. In other words, all the abilities survivors have. The survivors aren't immune, they just mutated in a specific way. Luckily for them, the mutation doesn't mess with their brains, or at the very least, it doesn't make them insane. Either they'll never know that they're infected and nothing will come of it, or they'll eventually mutate into something horrible, or they'll turn on humanity when they're hiding out in the supposedly safe, zombie-free sanctuary that CEDA has obviously set up. Downer Ending or what?
 * ... The Survivors don't regenerate. They can heal with first-aid kits, but it's not like a Survivor sitting all alone will regain health.

High pitched noises are what made the Special Infected....

 * ....Or more accurately, the long term state of rage caused by long term exposure to those noises. A bloodstream flooded with adrenaline put the virus in a very different environment to that of the common infected, who are pretty docile when left to their own devices. Sources of these noises that would last for roughly a week(the time it took for the special infected to appear, could be anything from several hours to a whole 7 days) would be fairly rare, which explains why the Survivors aren't drowning in an ocean of Boomer bile. As they mutated from common to special infected they became mostly desensitised to the enraging effect of high pitched noises, which is why they go after survivors over pipe bombs or car alarms but are still a dime a dozen during crescendo events. Witches are the result of a very different mutation, but with the same cause. The theories about Witches being pregnant or at a certain point in their menstrual cycles when infected may be related to this difference. The endless supply of Special Infected and the emergence of the Charger, Jockey and Spitter is a result of survivors and the military trying to use high pitched noises to contain the infected in the medium/long term and failing rather miserably.
 * Kind of a Brown Note for zombies?
 * But worse.
 * Green note, maybe? (Valve could hire Green Day for the sequels... ah, never mind.)

The idea of asymptomatic "Carriers" is just Valve's personal bridge.
While many people I've talked to are under the assumption, post Left 4 Dead 2, that all Survivors are asymptomatic Carriers of the Infection. I see it differently: that the idea of Carriers have been introduced to allow Valve to get rid of any Survivors, or groups of Survivors, that fans don't take a liking to. We've got eight Survivors now, and rumors that there's going to be a DLC in which the Midnight Riders are the Survivors. As it stands, that is twelve Survivors running around, not to mention any more DLC Survivors or sequel Survivors. So what do you do when you have 12+ Survivors running around, some of them liked less than others, some that have to die in order to prevent people wondering why so many Immunes seem to exist in close proximity to each other? Presto-chango, bingo-bango-bongo, Creator's Pet's of Left 4 Dead weren't Immune, they were Carriers, and they want you to kill them to avoid infecting others.

The virus is the same as that from Quarantine.
Not from REC, but the American remake. Remember all the empty rat cages in the uppermost apartment? All the rats were injected and released by the Doomsday cult to spread the virus. Also, it was likely released in more than one place at around the same time, since the recording mentions the cult's "brothers" in Houston. The only reason there were no Specials is that the mutation takes several days or weeks, and everyone in the movie had only recently been infected.
 * Doubt it. If you've seen the second movie then you know that the infection is  and that   AKA: in the movie,   In the game, the government hasn't a clue what to do about the virus besides isolating compromised areas, deffending islands that infected can't reach, and studying the immune for a possible cure.
 * 'course, the sequel could well be considered Discontinuity

The virus was developed by Mann Co.
In an attempt to replicate the abilities of The Team (a bit like Big Boss). Needless to say, it didn't quite work.
 * Saxton Hale was tired of Beating up wild animals and Hippies, so he decided to give Zombies a try. Then some of them escaped and the infection spread!

Keith is several different people
At one point Ellis talks about the time him and Keith "were stuck on top of a burning building and had to fight our way down like five floors of zombies to... Wait a second, I guess that was you guys". It's possible that Ellis really did know people who did all those things, but that it was a different person each time, and for some reason (maybe he's bad at remembering names) he attributes them all to 'Keith'.
 * Or maybe he just knew a lot of guys who just happened to be named "Keith". It's not too far-fetched- this troper knew at least five kids in his middle school named Jason.

The special infected are various stages of the disease.
Many people seem to assume that you become a special infected because of your lifestyle or some drugs you took before getting infected. However, I believe ALL infected evolve into special infected, and go from one shape to the other. The thing is, most of them don't live long enough to evolve, so you don't see as many special infected as regular ones. Here's how I think the infection evolves:

Step 1: Regular infected. Essentially, they're just very pissed off normal humans.

Step 2 (men): Hunter. The infection starts making the organs grow, starting with the muscles. This gives the infected much better athletic abilities, allowing them to jump and run more efficiently.

Step 2 (women): Witch. The infection acts differently due to hormones, but it's obvious that the infected's athletic abilities are improved.

Step 3: Jockey. The internal organs grow and harden. This makes the infected heavier and therefore lose some of their athletic abilities. However, they use what they have left to jump on survivors and let traps or other infected finish them off.

Step 4 (men): Smoker. The organs keep growing and hardening. This allows the infected to weaponize their organs (remember it's implied the smokers' tongue isn't really a tongue). Gas also starts building up inside the body, explaining the smoke.

Step 4 (women): Spitter. Interestingly. Even though the organs and muscles aren't as developed as the Smoker's, the Spitter already developed the bile that the Boomer will use, even though it acts differently. Possibly, this form actually comes after the Boomer, but that would mean the organs shrink before growing again, which just... doesn't seem logical (Then again, neither does a zombie apocalypse).

Step 5: Boomer. The organs grow exponentially, and the concentration of gas is reaching a critical point. The organs are now too big and heavy to be used as weapons and seriously slow down the infected (see, they're not fat, it's just that their organs got huge), and they have to resort to their last weapon: puking. The infected are the most vulnerable at this step, explaining why you see so few infected in the following phases.

Step 6: Charger. The organs quit growing, the gas starts to leave the body (you don't want to know how. Also, it explains why the underdeveloped arm is much smaller than the Boomer's arms) and the muscles grow again to keep up with the internal organs, improving their mobility.

Final step: Tank. The muscles have reached full maturity and the organs are hardened to the max, leading to this humongous, nigh unstoppable, highly muscular and tough beast. Note that since the disease is apparently a variation of rabies, it makes sense that the terminal phase would lead to a being of sheer anger.
 * One problem with this theory: The church guy. He generally morphs directly from human to Boomer, without going through the in-between stages.

Helen is The Director
. She seems like the type of person to start a Zombie Apocalypse, and really it's not much different from what she was doing before.
 * If not her, then one of her descendants.

Keith is the 8th Uncommon Infected, or it's someone the Survivors know.

 * Keith is the unluckiest son of a bitch in the world, according to Ellis' stories. According to Ellis' Facebook page, Keith was in the hospital during first infection from his turkey frying attempt. If the hospital turned first, Keith was probably infected. Since carriers are fairly rare, and Keith isn't related to Ellis, he's probably not immune. Due to his apparent indestructibility as a human, God only knows what a zombified Keith can do.
 * By the same token, according to Ellis' Facebook page, Ellis' mother was in the hospital with a flu- possibly the Green Flu. However, since she's a blood relative of Ellis', she may be immune.
 * It is mentioned in The Sacrifice Online Comic (Zoey's story, to be specific), however, that the immunity strain is usually inherited from the father, so Ellis' mother may not be safe from the virus.
 * Holy Shit! Ellis has a Facebook page?! Can a sister get a link?!

there are special carriers/survivors
If the characters of both Left 4 Dead and Left 4 Dead 2 are characters of the green flu, and, also making a guess here, the special infected are caused from mutations/strains in the green flu, isn't it possible in a million-to-one shot to become infected with a special infected strain, yet become a carrier? They would not exhibit the symptoms of the virus (rage, love of close spaces, attraction to noise, etc.) yet they would still be changed by it.

A future game will have the survivors having to deal with the military as well as zombies.
The plot will be kickstarted with three new survivors being picked up by the military, only to discover that they're going to be executed as suspected Carriers. However, one of the soldiers will protest the execution, and the four of them end up escaping together, now having to deal with being pursued by human opponents as well as the zombie hordes. This will lead to a Downer Ending where they basically decimate the occupying forces to the point where they don't have any chance of "containing the breakout" anymore... Although, of course, "containing the breakout" was likely impossible from the get-go.
 * That would quite possibly be the best game ever.
 * CALL VALVE. Or at the very least, e-mail Gabe. That's a damn good plot right there.

A Genius was responsible for the outbreaks
Because this is SO what one of them would do.

WONDER: Cure for all diseases!

Description: A machine pumps a bacteria into the water supply that destroys viruses...

Faults:... That replaces it with a one that could destroy the world with zombies, and it can evolve. But it does work.

Notes: Maybe I shouldn't have pumped it into the water supply of Philadelphia before actually realizing what it would do.

The Next Game will feature...
Keith, Chicago Ted, and one of the Midnight Riders. Chicago Ted will also be black.
 * As will Keith, who has been shanked for money by Klan members before.
 * Actually a Keith Model has been released... Keith's White. But he Could still be some religious minority the KKK hates. Maybe He's a Catholic like Ellis and they first met at church.
 * The fourth member of their merry band will be Nick's ex-wife.
 * Keith has been unlucky enough to go through many horrific things, but survive all of them. My personal opinion is that he is surviving the Apocalypse SOMEWHERE, but is alone, probably with no guns, and he is constantly missing out on any and all help from other people.

Keith is a tank
He's been infected for years. Ellis just didn't pay much attention.

Not only is Keith still alive, he's hooked up with other survivors and is telling Ellis stories.
For no particular reason other than it just seems so right to hear "I ever tell you about the time Ellis and I tried that fancy ninjitsu stuff? Ellis went and picked up some nun-chucks and started swinging it like they do in the movies. Nailed himself in the nose and the groin right after the other--he was talking in soprano for--" "Keith? Is now the best time?" "Okay..."

The helicopter leaving the hotel in the intro to Dead Center is the same one that picks up the survivors at the end of The Parish.
They appear to use the same model, down to the national decals in the same location. It would also turn the overarching story of Left 4 Dead 2 into a nicely closed circle, similar to that in Half-Life 2 (return to City 17) and Portal (return to earlier stages).
 * Not only that, but the helicopter is in fact dropping the Survivors off, and they're asking the helicopter to get back because it brought them back where they came from.

Keith is Ellis' Tyler Durden.
Ellis lives a daring life, and all those injuries and stories traumatized him to the point of splitting personalities completely. It explains the whole "wait a second, I guess that was you guys" confusion.

Scout isn't related to Ellis, he's related to Nick.
The evidence for Scout and Ellis' relation has always pretty much been 'hat.' But Nick and Scout are both from the Boston area and have criminal backgrounds. When Nick picks up a baseball bat, he will sometimes say "This brings back good memories." Those memories are of playing catch with his dear father, Scout, before he got carted off the jail/murdered. Also, Nick has never been confirmed as being from Boston, but he has a Bostonian accent.

Chicago Ted is not only real, the survivors are his apprentices
Essentially, the survivors are being guided and trained by the legendary zombie killer, Ted starting catching the infected himself so he can train others in his ways, and he got several 'routes' (campaign levels) to do it with.

the seeming sympathy towards the survivors from the director on lower difficulty levels, that is Ted controlling the flow of zombies and dropping in to leave weapons, med kits and ammunition for them, the difficulty chosen isn't relative to the player, it's how far trained they are by Ted, with beginner taking place actually earlier than expert, tougher zombies and more of them, the lack of med kits save for the crescendo levels and safe rooms is him sparing them for other, newer survival teams he comes across.

The fact that the blockades are always the opposite of the end of level door when you arrive there, and the survivors invariably blockade the way they came through, the course is run backwards and forwards by different teams of survivors all in his training, the messages on the wall with responses being added like an internet chat room are going to be re-read by those running the course again. the weapons having no variation between campaigns is because that is what he supplies to his trainees, the fact that all of the pipe bombs despite being found lying around are all of the same make and all have the high pitched siren on it, along with med kits and molotovs. Also, he always sends them off with minimal weapons between campaigns, dropping them off with a pistol and no further supplies, granted he always sets up something that has the supplies nearby for the survivors to make use of.

Left 4 dead 1 takes place after the second, there is more variations in the weapons in 2, Chicago Ted simply ran out of those weapon models when he moved north east due to the army bombing, he was leading survivors towards New Orleans, when the bridge was blown up along with most of the city he headed north-east, to Philadelphia, sticking near to the sea. He leads teams towards escape vehicles and eventually into military hands for evacuation, if they so choose. The survivors in Left 4 Dead 2 don't know each other at the start, yet they follow Teds courses through some of the campaigns, with them ultimately escaping, the survivors from the first fitting this a bit more closely, yet he still has safe houses leading to crescendo events that lead to their final escape, so he allows them to leave his training and escape with the army or CEDA.

This is a very minor one as well, but the way Ellis talks about Kieth with no sadness, maybe it isn't that Kieth's dead and he's gotten over it or he simply believes Kieth is alive, he's with another group under Ted's tutelage, I'm pretty sure with the punishment he has taken in his life he could be quite an asset.
 * This actually works surprisingly well, except for the problem that Left 4 Dead 2 takes place one week after Left 4 Dead 1, not before. Beyond this being established by external sources, it's supported in-game by The Passing, the bile "grenades" left behind at seemingly recently-abandoned CEDA sites that didn't exist in the original game, and the new kinds of special infected.
 * Okay, editing part of my idea to make it fit and acknowledging my mistake, also adding some more ideas, Chicago heads from Philadelphia down to the south, both finding new weapons, capturing the new special infected for his courses and training more survivors as he goes, he now starts using melee weapons also, realizing that teaching survivors about improvised weapons is very useful, the fact that in left 4 dead 2 has less ammo piles per map (from what I've been told and from witnessing) is both to encourage usage of different weapons (so they are proficient in every way) and encouraging usage of melee.

Having more models of weapons is due to him either finding them or his reputation is getting some support (in way of him being supplied), he continually adds any new mutations (not the game modes, special infected) so that people being trained by him are aware of those threats and have strategies to deal with them, also the fact that both groups of survivors, despite having never met, have similar names for special infected, at the start of left 4 dead 2 the survivors have never met and describe the SI by their attributes, then they call them by name as it goes on, thus picking it up from other survivors who run the courses.

The Tank and the Witch were both caused by anti-depressants.
In Real Life, depressed people are often either weepy or angry. While the two aren't mutually exclusive, many people go to one of two extremes. Generally speaking, women are more likely be sad much of the time, whereas men tends towards extreme irritability. The Virus interacted not only with the victim's anti-depressants, but also with their hormones. If more estrogen then testosterone was present when the person was bitten, they turn into a witch. And if more testosterone was present, they become a Tank.

The survivors aren't getting killed by the military; they're being conscripted.
This is an unprecedented threat; containing the infection seems next to impossible, and  isn't likely to completely turn the tide. They need to know how to fight these things effectively, and that's where the survivors come into play. Without the sort of resources the military has, they've still managed to survive this long; hopefully they can teach others how to fight. And then, of course, there's the possibility that they're immune... making them even more valuable as defenders.

Of course, they could also aid research on resisting the infection, or they could still be deemed dangerous "Carriers"... something the military can use to pressure them into service. Continue the fight, or get executed as "Carriers" (or, alternately, go through various experiments with or without their permission)... not much of a choice, is it?

The title refers to the survivors leaving each other for dead, not the evacs.
Think about it. In the sequel, you have Magic Defibrillators, but in Real Life, they simply do not work that way. The survivors don't die when they collapse, their heart begins beating erratically, and the defib simply stabilizes it so they can get back up. When you don't have a defib, they just get Left 4 Dead because there's simply nothing they can do anymore. When you get them out of safehouses, it's a literal case of He's Just Hiding. They regained consciousness, and managed to drag themselves to a closet where they locked themselves in.

None of the original survivors died at all in The Passing.
not really dead,  just sleeping after kicking the asses of any zombies dumb enough to think they could take   on. All that zombie-killing to protect yourself is tiring work after all. The blood? It's, uh, a Jockey's.

The AI Director's name is Ed.
In the Passing, a segment of graffiti in the Underground says that Ed is king of the zombies. Harmless Shaun of the Dead reference "Who died and made you king of the fucking zombies?!" or the TRUTH?! Effectively, this could mean that all this stuff is a scam gone horribly awry.

Nick and Francis are former members of the same gang.
Although they never met they had both been affiliated with the same gang at some point, if the design on Francis's tattoos and Nick's ring are anything to go by.

The Survivor's Preferred Weapons.

 * Louis: Machine Gun or M60
 * Bill: M16
 * Francis: Pump Shotgun
 * Zoey: Sniper Rifle or Guns Akimbo


 * Coach: Combat Shotgun or Chainsaw
 * Ellis: Auto Shotgun or Grenade Launcher
 * Nick: Pistols or Ninja Sword
 * Rochelle: M16 or Fire Axe
 * Ellis bot will actually always go for a sniper rifle.
 * The Left 4 Dead 2 bots usually select the best weapon available. If they have a choice between an Uzi, a Mossberg shotgun, or an M16, they'll go for the M16, every time. The Left 4 Dead 1 bots seem to have preferences for one weapon or another, though.
 * Confirmed. Left 4 Dead 1 bots are scripted so that they always grab the same weapon at the start of a level, every time. In the first campaign, No Mercy, if you go through the apartments to get to the subway, the survivors will be carrying whatever weapons they were using at the end of The Apartments. However, if you skip the apartments and go straight to the subway, Bill and Louis will always grab the assault rifle, Zoey will grab a Hunting Rifle, and Francis will grab the automatic shotgun. If you find a weapons stash later in the level, they may grab a different weapon (but even then, they seem to have preferences).

Ellis is his last name. His first name is Calvin.
For whatever reason, Ellis started going by his last name. I'm not sure why, but this just completes both stories for me in a way I can't really describe. Also, Keith is Hobbes.
 * Well this troper has to say, that even though her last name is Ellis (And lives in the South to boot), it is still not an uncommon first name. Besides, while you are in the elevator in Dead Center, Ellis goes into a long explanation of his name stating: "You can call me Ellis, or Elle. But actually I would prefer it if you didn't call me Elle cause that's kind of a girl's name..." You don't give nicknames to your last name, that's just awkward...
 * On the contrary, consider "Smitty" (a nickname for people named Smith) and Star Trek's Montgomery "Scotty" Scott. The real challenge here is figuring out how Calvin, an Ohioan, would've picked up Ellis's thick Southern accent.

Everything is a hallucination.
And all the survivors are unstoppable mass murderers, hyped up on pills. Rather than the infected being zombies, they're normal people seen through the eyes of survivors on a drug high. The military is really the military, and the evacuations are actually happening, but for a different reason. To get away from you. The only reason the survivors ever get rescued is because the saviors haven't actually SEEN what the unstoppable menace of the 'survivors' looks like.
 * They must be focused on other things during it then, considering more often than not they're sitting there in the boat/helicopter/whathaveyou screaming "GET IN THE boat/helicopter/whathaveyou DAGNABBIT". You think they'd notice the survivors slaughtering all those people on the way.

Keith doesn't exist. Everything he did was done by Ellis.
Ellis is too shy to admit he did a lot of the stupid things that "Keith" did. He doesn't want the survivors to think that he's an idiot, not realizing that they really don't care and like him the way he is. Instead of regaling them with tales of what he's done in his stupider moments, he tells the stories as things he's done with "Keith", hoping to keep morale up without appearing too much of a reckless thrillseeker. "Ellis: "Did I ever tell you about the time me and Keith were stuck on top of a burning building and had to fight our way down through a bunch of zombies? ...Oh wait, that was you guys, man I can't wait to tell Keith!""
 * This could be proven by one of his stories:

The fact that he mixed up one of the outlandish tales about Keith with something he really did may mean that everything he said previously isn't entirely a lie. Keith may actually exist, but he might've been the one who watched Ellis do the stupid stuff.
 * Jossed by Word of God; Gabe Newell said that Keith is real.

CEDA and the military are intentionally moving ahead of bands of immune survivors.
It took the Left 4 Dead 2 survivors far too long to get noticed or get a message out. The military likely has UAVs and reconnaisance planes flying off of carriers nonstop - why wouldn't they pick them up before then? Here's my theory: They're intentionally leaving them behind, hoping that they'll either wipe out the zombies in an area, or if they don't, make the zombies clump together for an easy airstrike. Cruel... but pragmatic. How many zombies do you wipe out on average in one campaign? All you'd need is one immune human with the ability to use a gun per 300 zombies.

Coach is Doc Louis.
After, he moved down south and got a job at a high school. After 10 years or so, when the Zombie Apocalypse hit, he'd gone bald and shaved off his mustache. Note that he maintains his love of chocolate.

The original survivors taught the new ones the names they use for the special infected.
Pretty simple, during an unseen part of The Passing, the new survivors ask about the special infected using like "The spitting one" and "That fat guy," and the original survivors told them what they call them. Also, when the millitary took the survivors at the end of Blood Harvest, they asked them about what they called the zombies, and word got around that the fat ones are called Boomers. That's where the big sign in the first part of Dead Center comes from.
 * This may be Jossed since certain materials show that CEDA was releasing info on the special infected; doing tests on them and putting out a poster isn't much, but when said poster has the special's nickname on it and some useful tactics/info, and is all over the infected area's, that's enough. Further, the survivors call the specials several different things, which means they aren't as attached to those names because they didn't invent them, or that those are just the main ones we know, and they aren't as standardized as it seems.

The Last Stand is a non-canon Downer Ending variant of an unfinished campaign that would bridge between Death Toll and Dead Air.
Valve initially planned to make bridges between all existng campaigns for DLC. since gamefiles and commentary confirmed the copter crashed between No Mercy and Death Toll, they decided to write a campaign where the survivors would have to get gas for John Slater's boat (ala Hard Rain) but Slater or his wife turns zombie and as a result the boat crashes and the survivors have to start the lighthouse generater for a mid campaign finale (called a Climax) which would be the first of many campaigns to use this new format. A boat rescues the survivors and drops them off in Newburg where they have to get to the rooftops because the street-level amount of zombies is too great. the campaign ends with a mini finale of sorts featuring an uncommon infected.

Coupled with the inability to contact Jim French, The ammount of coding required would be too much and as a result it was scrapped, though most of the content ended up in Left 4 Dead 2. the Climax featurre (While purely speculative on this troper's part) probably was scrapped because after facing such a great challenge the finale would not only be (for lack of a better word) anticlimactic.

This is more of a WMG on Valve's plans (gone wrong...ish) than a WMG on the game itself, however it can be applied as such.

I imagine it was planned something like this:


 * Slater pulling away from a dock as the survivors go to get tanks of gas. However, unlike Hard Rain, the entire thing will be like scavenger mode getting gas and bringing it to the boat. due to slater not moving the boat before getting gas he or his wife is bitten.
 * After getting to the boat, you are treated to a rescue style ending and the next level begins like a campaign beginning. Survivors make their way to a lighthouse from down the road which ends at a lighthouse. Pull the lever, start the lighthouse, and experience a double length finale called a climax. They get rescued by a new boat in the end (on a dock by the level's begining). New lines would have helped further develop the characters and would have been badass:
 * Bill: Dead end.
 * Zoey: It can't end like this!
 * Louis: Don't worry Zoey we can start the lighthouse generator, that will get us some attention.
 * Francis: Yeah, the unwanted type.
 * Bill: I know it looks grim, but I don't see any other option. Either we wait here and die or call hell to our doorstep for just a faint glimmer of hope. Either way, we're screwed so let me make this clear: I am not gonna go out silently in the night damn it!
 * Francis: Hell yeah!
 * The next level will be in Newberg and somehow you get to the greenhouse in the end.

The survivors are sin eaters.
Before they are called into bargains, at least, before they start to die.

Come on. They can see the auras of people and how long they have left. The reason zombies don't have an aura is because...They are zombies. When they first die, they are brought back to life.

I keep having this idea for an exchange that would prove this.

"Ellis: Hey, y'all. I keep seeing these weird...Auras around you. What the hell is going on? Coach: I see them too! I thought I was going crazy! Nick: I knew I wasn't really drunk during that night in Vegas!"

I wonder if Left 4 Dead 3 will be about how the supernatural population is getting on during the zombie attack. Place your bets on what races are going to be in Left 4 Dead 3!


 * If one small Krewe with no clue how to use their powers can take out so many zombies, most of the supernatural population could wipe the floor with them.

Canonically, none of the special infected have ever been killed.

 * Think about it, in the intros, none of them are ever destroyed, the survivors just slow them down or run away. Ingame, the survivors die regularily, but there is no reaction when they come back, so canonically, the survivors never die. So why is there no reaction to hundreds of zombies looking exactly the same? Because they're the same person, and they haven't actually died canonically. It's actually just the same five from the first game, joining up with the new three from the new game to take down the new survivors. With the female boomer, it's possible that there is more than one of each type of special infected, but the character designs we see are the specific ones hunting down the survivors.
 * Jossed. The Hunter dies in the Left 4 Dead 1 intro. It might not have been the intro, but some canonical material. Also, the smoker took time to change his shirt between the two games?
 * Also the bottom half of a Boomer can be seen right at the beginning of the intro to Left 4 Dead 1.

All the survivors you play are dead by the end of Left 4 Dead 2
It's fairly obvious how the new survivors will die after being saved by the military. With them killing so many zombies with melee weapons they will most likely get some infected blood in their eyes, mouth, or open wounds which would turn them into carriers. This will give the military a reason to shoot them. Although with surviving through hell, it's unknown if the survivors from both games could actually survive.
 * Lets start with the original survivors. Aside from all the Left 4 Dead 1 survivors are alive in The Passing, but occasionally, at the end of the campaign Louis will mention he has a good feeling he will see the Left 4 Dead 2 survivors again, to which Zoey replies, "Where at the mall?" The Left 4 Dead survivors will go to the mall from Dead Center and realize there's no rescue, where either they die or continue to the hotel, where they may eventually die.

Witches cry because their eyes take in too much light.

 * just think about it. they cry because any amount of light hurts their eyes. they are harder to startle with your flashlights off, and even harder to startle during the day. the only reason I see is that any amount of light hurts their eyes. hence, why they run away after killing people. they want to get somewhere nice and dark.

The reason for the Bridal Witch is...

 * Keith ran away from the wedding, leaving the blushing bride at the altar. She remained there for a few hours heartbroken, then the infection came in and... the rest is history.

The Director is a Special Infected.

 * Like the other infected, he has had one specific part of his anatomy mutated to an extreme extent. Which part? his brain. He's super-intelligent, but still driven absolutely insane by The Virus and dedicated to wiping out mankind, either due to simple hatred or that he believes the Infected are the next step in human evolution. He is also one of the most hideous Infected, with tumours growths of brain tissue hanging down from his head, giving the impression of a living blob of highly sensitive flesh with limbs sticking out underneath. Left 4 Dead 3 will end with the Survivors fighting their way into his fortress, which is guarded by more Special Infected than ever, and blowing the Director to pieces, reducing the remaining Infected in the world to misguided, crazed animals, like wild dogs, which will easily be hunted down.

Florida is still safe.

 * Seems like Louisiana is already very unsafe from the infection, but what about Southeast Florida? If other continents military have started taking steps against the infection, one of the best places to begin an assault would be Florida. It's extended from the mainland, is far south, and is flatter than most other states. The flatness would be good for fighting the zombies, as you can see farther and fire farther while being perched on a very tall roof. Its proximity to the ocean and distance from the initial infection mean that few infected would go there, allowing the few stuck there to make a good defense. I find it likely that Florida will make it into Left 4 Dead 3, assuming the survivors are still in America and still traveling East.
 * I'm hoping the third game will be set a month or two later and farther north. Assuming the evac station in Nawlins hasn't been overwhelmed, the game could start in Minnesota, where snowstorms and ice are potential hazards. The rest of the game would be traveling south along the Mississippi river, trying to reach salvation in Louisiana.
 * The evac station in Nawlins has been overrun. Dialogue plainly states that the helicopter that the survivors are evac'ed on will be the last. Apologies to the OP, but Southeast Florida hasn't escaped either... the map in the hotel in Left 4 Dead 2 has Fort Lauderdale crossed out (overrun), and since Ft. Lauderdale is less than 30 miles from Miami, it's probably safe to assume that Miami (and by extension, most of Florida) is overrun or compromised. Of course, the map only covers the eastern seaboard; we have no idea about the status of Canada, or any states further west than Louisiana. In my opinion, the safest places would be Alaska or the northern reaches of Canada; unless the infection somehow confers resistance to the cold, the infected (who are operating only on animal instincts at this point) would not think to wear additional protection against the cold, and exposure would probably kill them before they could get that far north.

The Special Infected are actually children.

 * Think about it for a second. You never see any children, infected or not. So, this troper believes that the special infected are infected children, as many of them seem at least somewhat immature. The Hunters pounce on people, like a little child would pounce on their parents. The Witch cries a lot, and violently attacks anyone who interrupts her. Spitters and Boomers vomit a lot. So, it seems that the special infected are children, who quickly grew to huge proportions due to the infection. The wedding witch? A bridesmaid or flower girl. They can have ornate and fancy dresses too, and are often daughters. Obviously, Jockeys didn't do too much growing when they got it, they just went-very-insane. Also, young children like to play "horsie" with their parents or older siblings. Also explains why the Smoker lashes his tongue at you: He was at that age when boys have dreams of being astronauts or cowboys. Roping horses in his daydreams transformed into roping people with a long tongue thing.
 * Possibly Jossed. It's been confirmed by Word of God that the infection is always deadly, though it probably takes a longer time for the Special Infected to die. Think about it: The virus is a mutant form of rabies (according to Word of God again). The symptoms of rabies in humans include irritability, headache, pain, itching or a twitching at the infection site, and fever. As the disease progresses, muscle spasms in the throat and respiratory tract affect breathing, and the sufferer may have difficulty swallowing, the combination of which can produce the trademark "foaming at the mouth". Further symptoms of rabies as it progresses are hallucinations, convulsions, seizures, paralysis, and eventually death. And, in fact, if you use the notarget cheat and just watch, sometimes an infected will simply lie down and die for no apparent reason. Children don't have as fully-developed an immune system as adults, so probably the reason you don't see any children in the game is that they died a lot more quickly, or died instead of "turning".
 * Um, downer time but maybe there are no kids 'cause their parents killed them? Think about it, it the world was going to Hell and you started getting sick, would you want your kids to go through that? Now imagine the parents actually turning and killing their children by ripping them to shreds. It's highly possible that the parents decide to put their kids out of their misery before they officially turned and had to suffer being an undead.
 * It could be a case of women-and-children-first in evacuations.

Keith isn't real. He's Ellis's way of coping with the apocalypse.

 * Ellis has convinced himself that Keith existed, and went through so many potentially fatal injuries and events that no human could possibly survive. That's because they couldn't survive. The reason Ellis convinced himself that Keith had amazing durability is to give himself hope. He believes that, if Keith could survive drowning, immolation and being shredded by a lawnmower, then he and his teammates can survive a zombie apocalypse
 * Jossed. Word of God says Keith is real. However, that doesn't mean Ellis didn't make those stories up.

The 4 original playable Special Infected were a prequel Survivor team.
It works like this. One day, somewhere in the heart of Pennsylvania, 4 ordinary folks (a bodybuilder, a parkour-jumper, a chain smoker, and a very obese alcoholic) were suddenly thrust into the middle of an apocalypse the likes of which only captured in movies: hordes of zombies, straight from CEDA's quarantine facilities. Using only their wits, their teamwork, and several molotovs, they valiantly fought off the hordes of zombies. But their leader, the parkour-jumper known as "Hunter", wanted more. He wanted to end the nightmare by going to the source: a nearby hospital, the first to be used by CEDA to contain the infected.

Many zombies are slaughtered in the corridors of the establishment, until finally, they come upon the AI Director, a madman whose very brain cells are iradiated by the virus, allowing him total control over its properties. He quickly subdues the Survivors and infects them, transforming them into the twisted shapes they take today. This, for some sick reason, pleased the Director, so he decided to "make" more of these "Special Infected". Not only that, he's gotten even more creative, and "created" abberant mutants from the original 4 (Jockey from Hunter, Spitter from Boomer, Charger from Tank). Not only that, but he has taken the "game" the first Survivors played a step further, and continued it. The Director has now started manipulating events and hordes so that, all over the Infection's area, different bands of 4 team up in a futile attempt to fight off the Infection.
 * Then explain the Witch.
 * The "Witch" was a small NPC, a cranky old woman who had once been Hunter's landlady. She was subdued by the Infected early in the siege, but her death inspired Hunter to take to the offensive in the first place. She was later used as an "experiment" to produce a special infected, originally to further decimate the uninfected. Once the AI Director got a hold of the 4 original survivors, however, the Witches went a new direction, and took their place by the Special Infected's side.

There is only one Hunter, one Smoker, one Jockey, one Charger, and one Spitter.
All of the ones you fight are the same one. When they "die," their bodies just shut down and rapidly regenerate. The Hunter and Smoker migrated to Savannah while chasing the Left 4 Dead 1 survivors, where they find the new Survivors and begin hunting them. Boomers, Witches, and Tanks are produced from fairly common strains of the virus, so there's plenty of them going around.

The Left 4 Dead 3 Survivors will meet the Survivors from both of the earlier games, each lacking a member.
They'll run into the Left 4 Dead 2 crew (minus Rochelle) on their trek to safety, and there'll be DLC where they have to, with Francis, hold off an infinite horde while Zoey takes the still-crippled Louis up an elevator, and then escape while Francis stays behind to ensure their safety. This trend continues over the series until the first four groups only have one member, and they team up to cleanse the plague once and for all. (That would require a lot of games, though.)

The board game Pandemic is an alternate continuity of Left 4 Dead.
As the mutant rabies strain spread beyond America, and as CEDA continued to fail to contain it, the American government disbanded the branch and returned emergency powers to FEMA and research authority to the CDC. As America continued to struggle against the disease variant strains appeared in other countries, which were rapidly overrun. As the game opens, the CDC has identified four main strains that need to be cured.

The cubes in the game represent both a disease and a number of zombies. "Curing" the diseases involves not only your game role, but a full army squad; that's why your game pieces are so big and why traveling from city to city takes so much effort. Outbreaks not only represent the disease spreading from city to city but also zombies breaking through the meager defenses left. Further, the game differentiates between "curing" a disease and "eradicating" it because the remaining zombies are still a threat.

The mutating strain from the expansion is the Special Infected.
 * But is Pandemic related to all this?
 * Clearly, the next game must be set in Madagascar, then. Shutting down everything didn't save them this time.

The military rescues the survivors at the end of The Parish because everyone left is either a carrier, or true immune.
Or a badass, like Keith and Chicago Ted.

Keith's last name is Stone.
And he survived all his previous predicaments through sheer smoothness.

The AI Director is a Survivor who practices Necromancy.
He/she found he could control the zombies and direct them wherever he wanted. Instead of using this power for a good purpose (i.e. keeping them away from the other survivors), he/she has instead used them to attack the living for an undisclosed reason.

Francis and Jayne Cobb are the same person.
No, I can't explain this, but they are exactly the same character, right down to the beard.
 * Time travel!
 * Identical Grandson. Also, the government kept samples of the zombification virus even after the crisis ended, and it was used hundreds of years later in the experiment that accidentally made the Reavers.
 * Pax was developed to sedate the population. There's no the way the government'd figure the Infection to be useful to that effect.

New special infected!
Just shooting the breeze here.
 * 1: A Squishy Wizard with an enlarged cranium. Hyperintelligent, commands other classes.
 * 2: A Mook Maker with a hugely distented stomach. Fan Disservice.
 * 3: Combine mooks of some sort.
 * 4. An Infected that crawls through the sewers and can grab survivors when they get close enough to a manhole or gutter.

The Carriers are another breed of Special Infected.
As mentioned above, they're tougher and stronger than any normal human. Despite the fact that Rochelle and Nick aren't really familiar with guns, they have excellent aim, which points at improved motor control and vision. Plus they have extremely, inhumanly well-developed pack and homing instincts. I'd posit that not only are carriers like the player-characters special infected, the military is interested in them because their version of the virus is one that they want to duplicate for use in a super-soldier program.
 * Actually, Nick is very familiar with guns, and in-game dialogue from Ellis when Nick kills a Special Infected hints at this. "Why am I not surprised that you're good with guns?" Since he is a riverboat gambler (and was originally intended to be an escaped convict who had stolen a nice suit when he escaped), my WMG is that Nick spent some time as an enforcer for an organized crime group. ("Brains wash out. Swamp water doesn't. Don't ask me how I know that.")
 * Also, it's far more likely that the military started out killing immune carriers because they were a source to spread the infection, and then reversed its policy when they realized that the carriers might be their only chance to find a way to inoculate the non-immune people who had not yet been infected. It's also more likely that, even if they're not killing the immune carriers anymore, they'll treat them like "illegal combatants in a war for the survival of the human race" (which means the survivors have NO rights, except the right to be experimented on.)

Louis is a Carrier.
Possibly huge spoilers coming up, especially if you haven't read the comic yet.

As seen in the comic, Louis is bitten by a zombie in the early days of the infection, though he hasn't turned, suggesting he's either immune or has become a carrier. This could also mean the Mercy Hospital pilot became infected via Louis somehow, and may have also caused the other escape vehicles to... not escape. Francis mentions how the plane from Dead Air crashed, suggesting Louis infeced him too, and that the boat pilot from Death toll left the survivors to die. It could've been possible the boat pilot's wife, Amanda, was infected by Louis accidently, and the pilot, suffering from grief of the loss of his love and not wanting to be infected himself, kicks the survivors off the boat. The other three survivors are properly immune to not get infected, and the new survivors are too distant from Louis in The Passing to get infected anyways. The soldier in the comic driving the APC also takes the survivors of Millhaven instead of Echo, which may be because said soldier noticed Louis's bandaged arm, and didn't want to take any chances. In fact, this may actually become a plot point looking at things as they are.
 * It seems that ALL of the original survivors are carriers.
 * Worse than that, ALL the immune are carriers, since Green Flu was at one point, airborne. It's unlikely that there are any uninfected immune left, meaning trouble for the Left 4 Dead 2 survivors who were last seen with the military. But hey, at least Zoey, Louis and Francis are safe.
 * Correct. If you have been exposed but show no signs of infection, you are a carrier. If you got evacuated and were never exposed, there's no way of knowing whether you are a potential carrier or not without exposing you... and I wouldn't want to find out.

Bill isn't dead.

 * We don't see him die in the comic, merely prepare to take on 3 tanks. We don't see whomever sacrifices themselves die in the actual game, they merely get incapped, then fade to black. In the passing, he hasn't even MOVED. As such, I think he didn't die. It's notable that none of the 4 new survivors bothers to check if he's dead, and the survivors can recover from having that much blood pounded out of them. Bill's merely napping. (It's a Bolivian Army Ending :P)
 * Word of God: It's worse than that, [ dead, Jim.
 * Actually, in the game you can find his corpse. Last time I checked, he looked pretty dead to me.

Implants turn people into Witches.

 * Think about it. In part Four of the comic there is an entire yacht filled with bikini-clad witches, in a room that looks like a photo shoot. They all have rather large chests in proportion to their body size. The Infection could react with the implants, and it spreads throughout the bloodstream giving them a lower tolerance for agitation as a side effect. (Kinda like how you get really cranky when sick with two different things at once)

The Special Infected are:

 * Boomer
 * Gabe Newell. Oops! I delayed Half-Life 2: Episode 3 again!
 * Hunter
 * Garrett. I mean look at him. He has the hood, the face is pretty close to matching up, and he appears to lurk in shadows. So this all adds up.
 * Alternatively, he is an Assassin. That would explain the hood, the aptitude at killing at close range, and the Le Parkour skills.

Keith is real and all the stories are true, because being immune gives you a Healing Factor
Nick can throw a Molotov cocktail at his feet and run around in the flames for a while and still only lose 20 health. Coach can be grabbed by a Charger, shoved out a window, and slammed repeatedly into the floor and ceiling, then dust himself off and walk away. Rochelle can be hit in the head with a flying Dumpster and all she needs is to be picked up off the ground and then she can move under her own power. And when any of them is too pained to keep moving, they tend to solve this problem by gobbling an entire bottle of painkillers, which not only has no ill effects, they completely wear off after a couple of minutes. Does surviving third-degree burns over the third-degree burns over 90% of his body, being blown up by experimental Army bombs, or drowning in the Tunnel of Love really look that improbably by comparison?

Painis Cupcake is the final special infected in the game
He will eat you. :D

The virus is a weapon created by the military, Gone Horribly Wrong.
Specifically, it was intended to create Super Soldiers. The first test subject mutated into the Tank, who proved much too powerful for the researchers to control. They tried to restrain it, but unfortunately the Tank escaped the research facility and rampaged across the country, infecting everything it came in contact with. (An unintended side-effect of the virus is that it's incredibly contagious.)

Naturally, the virus affects everybody differently, hence why there's such a wide variety of Special Infected. Mutation into a Tank is considered rare. Going from this theory...

Canonically, there is only one Tank per game.
You don't actually ever kill it. You just knock it out for a while. When it wakes up, it starts to chase after you again, much like the Nemesis.
 * I don't think Bill was hallucinating when he three at once.
 * And I doubt the other survivors were either, and there were originally four, I think. But on the flip side, the troper did say "per-game" so it could have been the Left 4 Dead 1 and 2 tank working with the two from the next two sequels.
 * Alternatively, there's only one tank per campaign, and you only actually kill it in the Finale.
 * Actually, Bill WAS slugged right in the head before seeing "Three" Tanks.
 * No way, I've seen more than one Tank at a time in finales before. (One of them was dead, but that's still proof).
 * Although it could be true that sometimes Tanks get up and come after you, if you kill them with 1st Tier weapons, for example…
 * There's a special version of the Tank in the docks section of "The Sacrifice" that appears to be an infected Marine. So there's definitely more than one out there.
 * In the finale of Swamp Fever, the second wave of zombies will always include two Tanks at once. Last long enough on a Survival round, and eventually multiple Tanks will start spawning at once. On a "what if" note, though, it'd be interesting if each Tank you faced throughout the campaigns was the same Tank, who starts catching on to their tactics and showing up in more dangerous areas like enclosed spaces and car lots. It eventually culminates in him wising up to the survivors enough that he punches a boulder from the top of a cliff at them in Cold Stream before they even see him.

Keith is going to show up in Left 4 Dead 3 or some DLC as an Infected.
He'll look like a Common Infected but will be very, very hard to kill.

Zoey backwards is Eezo!
Coincidence?! I think not!!

Zoey and Rochelle have an XY karyotype
Why? The comic "The Sacrifice" establishes that the Carrier gene is passed down on the father's side... but the only genetic information solely provided by fathers is the Y chromosome. For a trait to be passed down solely from the father's side, it would thus have to be on this chromosome. Given that the zombie apocalypse would likely cut them off from the hormones needed for transsexuals or Swyer Syndrome sufferers, Androgen Insensitivity Syndrome is the most likely cause of their being physically-female XY-types.
 * There goes Repopulating the species.

The Director is the GOOD GUY
The Survivors are clearly a threat to humanity, due to being carriers. However, it is clearly impossible to survive being bitten with such huge amounts of zombies, especially ones that fast. It is quite possible that the Director is simply an entity that, while not the cause of the infection, can control the zombies to an extent, basically telling them who to target. So he focuses them on carriers around the world in hopes of executing them before they can infect anyone else. This leaves people who haven't been bitten fairly safe, encountering very few zombies. All these uninfected survivors face little resistance, and can therefore get rescued quicker.

The Hunter is an Infected Assassin.
Because guys in hoods jumping around are just so common.
 * Have you ever seen hoodrats running around a city? They gotta be agile to run from cops, jumping over cars and fences and scrambling up building ladders, fire escapes and such to get out of trouble. Sure, they're probably not all Parkour enthusiasts, but still.

The Dummied Out Screamer Infected is patient zero.
Originally there was going to be a Special Infected called the Screamer, a gaunt man in a straitjacket that could scream and summon a horde of Common Infected. He's actually the first ever infected, and was being kept in Mercy Hospital, hence the straitjacket. The reason he's not in game is that he either died or was killed before there were enough infected for him to summon.

The infection didn't actually rot the Hunter's eyes out.
In addition to enhancing athletic abilities, the Hunter strain of the virus actually causes the body to break out in a heavily irritating rash, as shown by the sequel's Hunter's exposed arms. This also affected its face, and caused extremely irritating swelling around the eyes until, mentally unstable from the infection, it clawed out its own eyes.

Amanda Slater was dead to begin with.
Her husband is the only one who is heard speaking. It could be that she turned and John was forced to kill her. Overcome with grief, He keeps her corpse with him and talks to her as if she was still alive.
 * Jossed. According to the comic, the survivors saw them both.

The two teams will meet again in the Florida Keys.
Instead of taking the Left 4 Dead 2 survivors to an army base to study them, they are dropped on a cruise ship in the middle of the Gulf of Mexico, to live out the rest of their lives until they die of starvation or kill each other. Nick convinces his new family to grab a lifeboat and try and escape. They end up on on of the Keys, and after some exploration, they find their friends from the first game, and Ellis finally gets the chance to propose to Zoey. (Whether or not she accepts is up to the writers).
 * Although this is probably unlikely due to the creation of Dead Island and the whole "zombies in tropical paradise" idea may look like it's been done already, as someone who lives in Florida, I can tell you I'd love that. Or, better idea, their "paradise island" is overrun and they have to fight their way back up through Florida. A level devoted to the space center, murderous wildlife including both gators and snakes, senior citizen zombies, and enough pills/oxycodone you can OD on, anyone?

Left 4 Dead and Assassin's Creed take place in the same 'verse.
The Green Flu was created by the Templars with the intention of making its victims docile and compliant, but it went badly wrong. The Carrier gene is actually the Assassin gene- as well as providing immunity, it gives the Survivors a form of Eagle Vision which manifests as auras around their fellow survivors/useful items, and tints all alarmed cars red.
 * This explains why evacuation was so poor. The influential Templars focussed the government's resources on getting *them* out of danger. It would contradict the theory that Hunters are the result of Infected Assassins, however

The playable characters are all Dead to Begin With and are in purgatory.
Many of them have lived less than noble lives, why not all? They all died somehow, and their experience of Purgatory blends seamlessly with their perception of their daily lives. "Two weeks after first infection" is really "two weeks after death" for these guys. When Bill was Killed Off for Real, since he sacrificed himself to save the others, he goes straight to heaven, but the body the other survivors see was left there in purgatory. The other survivors have to either work together and reach the gate to heaven, or somehow atone for their misdeeds in life.
 * And that explains why they respawn! I love it!
 * And just for fun, i going to say what they did, and how they died!
 * Bill: He didn't kill people because he was following orders, he did it because he enjoyed it. He died on the operating table during that knee surgery.
 * Francis: Theft, murder, typical gang activities. Was shot by a police officer, this is why he hates cops.
 * Louis: What could happy, Friendly Louis do? How about embezzlement? He dies of a pill overdose (antidepressants this time, not painkillers).
 * Alternatively, he finally snapped while at work and good 'ol friendly Louis decided to show all those who laughed at him exactly what those lunch hours at the rifle range were good for.
 * Zoey: She neglected her education, and most likely caused the rift between her parents. Died of a meningitis infection in college.
 * Coach: Inappropriate contact with an underage student of his. He suffers a heart attack.
 * Nick: Again, we all know what he did. He got shivved in prison, if you look at his chest, you can see a bit of the scar.
 * Ooh, close, but no Medkit. Here's your consolation pills. The injury on Nick's chest is canonically described as nail marks from a woman. So, I'll agree that our conman got shiv'd in prison, but with an added bonus. He killed his ex-wife to get there, and the marks are from her attacking him in self-defense. It's also why Witches are particularly cruel to him.
 * Rochelle: She didn't care who she had to stomp on to get what she wanted, and she died of carbon monoxide poisoning while she slept.
 * Ellis: Wanna know why Keith suffered so much? Ellis forced him to do it. He drowned after driving home drunk one night. (Keith is still alive, but barely).

Building on the above: the survivors represent the Seven Deadly Sins.
Apart from an angelic guide who appears as Bill to the original survivors and Virgil to the second group, each survivor represents a specific cardinal sin.
 * Francis is wrathful.
 * Louis envies the other survivors for their experience, knowledge, and combat ability.
 * Zoey was slothful in college.
 * Coach is an obvious glutton.
 * Nick, the con artist, represents greed.
 * Rochelle pridefully holds herself above the other survivors and plans to use the apocalypse to enhance her career.
 * Ellis might not be the conventional sexual kind of lustful, but he's enthusiastic in a way that's kind of unseemly.

The survivors smell awful.
The Common Infected and Witches only attack Survivors when provoked. So why do the Hunters, Smokers and Boomers constantly hunt them? Because they have a strengthened sense of smell and the Survivors smell to them like Boomer Bile does to Common Infected. This explains why the Specials are always finding you and why Boomers vomit on you; they're trying to cover up your stench (it doesn't work).

Black people are immune to the Green Flu.
Not only do we not see any black Infected, but the number of black survivors has doubled between games. Thus, should there be a Left 4 Dead 3, all four survivors will be black. We don't see any black NPCs because most likely they had already fled to safety, or were overwhelmed.
 * Explain the white survivors then.
 * Jossed, look closely in Left 4 Dead 2, There's quite a fair number of black infected.

Francis's Last name is "Khapps".
So he's technically not lying when he says he is "the cops", it's just no one's ever asked him to spell it out.

Special Infected Origins
Most people become Common Infected, but they can become Special Infected if one of the following conditions is met. Gender is irrelevant to the process, as I take the view that there are male Witches and female Tanks, and Valve is just saving memory by not showing them, same as with female Turians. Should more than one condition be met, the higher form takes priority.
 * The Witch: Attempting to prevent a turn through sheer force of will and having sufficient willpower to do so
 * The result is an Infected who has the usual perpetual state of emotional fragility and hatred towards Humans but recalls their past life and personality. Thus they are grief-stricken at their current state, they attempt to scare away Survivors while they can still resist the rage and they are shocked out of their rage-state once they kill a Survivor by what they've done, so they flee, grief-stricken once more
 * Also, when the Infection is resisted, it consumes the host to fuel it's push towards domination. This ages the Witch considerably, which is why their fingernails and hair have grown so long
 * The Tank: Adolescents on the cusp of a growth spurt when the Infection strikes
 * Basically, right when all the hormane glands are set up and ready to get to work, the Infection bursts in and hijacks the growth spurt, putting everything into overdrive except for myostatin, which gets eliminated entirely
 * (http://en.wikipedia.org/wiki/Belgian_Blue) This is what I'm getting at
 * The Hunter: Being driven insane by the apocalypse, taking on a feral state
 * Once this feral state has been reached, the Human will be ignored by the Infected. This results in them getting into a routine of climbing somewhere high up from the still scary Infected in between feeding and sleeping. The Infection lays dormant in the feral during this time, enhancing the exercise they're giving their legs (while their nails/claws grow semi-naturally, at an increased rate)
 * This takes place over about week. The Infection is designed to quickly override an ordinary human mind, so when it encounters the basic beast-mind in the feral, it takes a longer time to instil the violent impulses towards humans, at which point the Hunter is born
 * The Smoker: Having cancer, of any kind, to any degree greater than minor
 * The Infection weakens a body's defenses to do it's work. This results in the cancer spreading throughout the infectee's body at the same time as the Infection. The Infection responds by regularly expelling most of cancerous cells from the body (causing the eponymous green smoke). The gas secreted by the Smoker is essentially the gaseous equivalent of mucus, except it contains cancer rather than germs
 * The rest of the cancer cells are directed by the Infection towards the tongue, where upon they are formed into a weird tumour relative of this (http://en.wikipedia.org/wiki/Cymothoa_exigua) thing. The end result being a large organ emerging from the mouth of which the Smoker has control
 * The Boomer: The Infection gaining entry to the pancreas early on in the change (harder than it sounds, the pancreas is hella out of the way)
 * The pancreas must be accessed quickly in order to trigger a response in the Infection that halts the rapid mind-changing process, replacing it with a slower process that doesn't degrade intelligence like with most Infected (but still evokes rage towards humans). This is necessary because Boomers are (IMO) the smartest Special Infected next to Witches, and they need to be to lead the others Infected in the attack (and survive to do so again)
 * Anyway, if the pancreas is accessed in time, the Infection seizes control of the enzyme-producing facilities, altering them to focus only on quickly producing large quantities of a new enzyme and dumping it into the stomach. This new enzyme reacts upon contact with human sweat to produce a very strong and very nasty smell, drawing the wrath of the Horde. The Boomer expands whenever he goes for a while without attacking a Survivor, as the enzyme is still being produced
 * Common Infected: Get the infection.
 * If none of these conditions are met, the Infection simply forces it's way into the infectee's brain and simultaneously destroys the memory centre, strengthens all of the senses and induces a massive migraine state.
 * The end result being that whenever a Survivor shoots their gun or shines their flashlight at an Infected, the Infected is overcome with pain and can think of no other solution than to attack the source. Even if the Survivor attempts to sneak past, they won't have showered in a long time, and they will smell horrific

Teen Titans is in the same 'verse, all the Titans are infected

 * Starfire is a flying Boomer.
 * Beast Boy is shape-shifting Smoker.
 * Robin is a very elusive Hunter.
 * Raven is a telekinetic Witch.
 * Cyborg is a Tank with a Laser Cannon.

There are two unique strains of the green flu virus.
The Airborne version works just like rabies, meaning that people do not know they are infected until about two weeks later. The Virus was actually released long before the "first infection" but that is only when people started showing symptoms. For this airborne virus, you have a 50% chance of survival, so initially there were many more survivors. The second strain is the one transferred via bite. It was lying dormant within the Airborne virus and was only awakened when they first strain managed to takeover the host body. The biting virus is much more infectious. Take for instance, Zoey's mom. She was smack dab in the middle of the infection site, yet she was immune to the airborne strain. When a zombie broke in and bit her, the bodily fluids strain got to her, and she was not immune. Both strains were derived from the same virus, and are already so similar microbiologists think they are one and the same.
 * Also, Boomer Bile is chock full of the biting strain, as well as pheromones to alert other infected "Hey! the infection can't get in, get over here and make an opening!" Boomer is like the only Infected not trying to actively kill you.

The contents of a Medkit are nothing more than a large roll of duct tape.
They just tape the wounds up and keep going.

Blood Harvest takes place in England.
After Dead Air, the Survivors fly away. It makes sense that they'd try and fly to an island in the hopes of escaping the Infection. England is Infection-free when they get there, but the quarantine eventually fails and they have to face the horde once again. This explains why Blood Harvest isn't set near an airport.

The Infected are frozen in their moments of infection
Think about it. 50% of the infected you see are leaning on a wall, or shambling aimlessly. When they were infected, they were frozen in their last moments of humanity until the survivors came in. It would also explain the Witch.

The Infection is caused by Chaos.
And the Special Infected are the chosen of the four gods. The Tanks and the Chargers are Chosen of Khorne, for fairly obvious reasons, namely being huge and full of rage. The Boomers and Smokers are Chosen of Nurgle, what with being covered in swellings and having abilities relating to poison and bile. The Spitters and Hunters are Chosen of Slaanesh, being faster and more agile in the Hunter's case and and really messed-up Fan Disservice /FetishFuel for the Spitter. That leaves the Jockeys and the Witches for Tzeentch. Both use uncoventional tactics and both are about using the Survivors as their own doom, with the Jockey directly controlling them and the Witch causing them to provoke an attack that is their own fault. The Green Flu is Chaos' first attack on humanity and will cause the God-Emperor to decide to begin his effort to lead humanity.

The virus isn't really a strain of rabies, they only THINK it is. It's actually yet another strain of Ebola - gone horribly, horribly wrong.
I know this is going against Word of God here, but hear me out. Originally, when the filovirus family was first discovered, based upon the early symptoms, it was actually given the misnomer of "stretched rabies". The intense headaches and bodily ache, the change in behavior, becoming sullen, listless, and then aggressive, lashing out at those who disturbed you, the inability to keep nearly anything down (food and water included), eventually the brain is actually "destroyed" as the organs shut down, leaving the basic life functions/brainstem intact. Now throw in a few other disturbing symptoms. Black vomit, which you can see some infected puking up during their idle animations. Deterioration of internal organs and connective tissues, massive bruising, and a skin rash of many little bumps across the body (easily visible on Hunters in L4D2). The deterioration of connective tissues and organs or the alteration of organs could explain the very easily-dismembered bodies, and the extreme changes some of the Special Infected undergo in their mutations - especially the Boomer considering the fondness Ebola has for the liver and hepatic system, thus inducing the bile. Also, the spleen, an organ used to filter deceased/dying blood cells out, could induce blood poisoning if shut down as it usually is under Ebola symptoms, and cause the pale, clammy-looking skin and such symptoms the Commons have. Ebola also changes the eyes, you will literally weep blood, they turn a very dark red. Several infected show this symptom as well. The disease spreads insanely fast, changes modes of transmissions constantly (see the fluid-borne Zaire strain vs. the more airborne Reston strain), and due to it jumping entirely different species of primates quite easily, could make the jump from livestock to humans just like rabies. The fatality rate can be anywhere from 25 to 90% depending on the strain, too, making this a highly lethal disease. And it has been known to do this all in the short span of just a few weeks, indeed, the fact that the disease is usually isolated to deep Africa and that it can burn through its victims so quickly before it passes on to another is usually considered the ONLY thing preventing a worldwide pandemic.

In short, ladies and gentlemen, this is no rabies with flu like infectivity. It's much, much worse.
 * As a troper who has just finished reading The Hot Zone, I agree fully with this WMG, at the cost of opening an entirely new can of paranoia for me.

The Infected don't know they're Infected and think the SURVIVORS are the infected.
How else would there be a wedding for a Witch?

Chicago Ted is dead.
He wasn't actually as badass as he thought he was, and was merely a boastful narcissitic, which lead to his downfall.