28 Days Later/WMG

Jim dies at the end.
After receiving a nasty gunshot wound to the stomach, the camera freezes as Jim's friends desperately try to rush him to a hospital. A caption appears with '28 days later...' written on, just like at the beginning. After a quick-cut summary of Jim being patched up at the hospital, he wakes up in a little house in the country with his friends. They're soon spotted by a plane sent by the outside world, with the possibility of rescue on the way. Notably, this last sequence is shot differently from the rest of the movie; the rest of the movie is shot with a 'rough' digital camera, but the epilogue sequence is shot with traditional film stock. Some believe this indicates that Jim didn't survive his injuries; this is his idea of 'paradise' after the weeks of torment he's just gone through. It helps that, in the original drafts of the script, and the alternate endings shot and, in some screenings and on the DVD, played after the credits, Jim did die; the film-makers decided this would be a bit too much of a Downer Ending.
 * Note the camera panning over them at the end. They've written "HELLO" on the hill, but the O is strangely cut out of the shot. That's what happens when you punch a priest...
 * The effect is driven home with a flash of these green pastures during the rescuscitation scene. While the viewer is still not sure if Selena was able to revive Jim, an idyllic landscape flashes on-screen; but it's upside-down and has HELL written on it in pristine white. It doesn't take WMG to see the effect was intended (moreso considering that the rescuscitation scene and the landscape are basically parts of Jim's REM dream. If noted, his awakening itself doesn't really convince the viewer he's not really dead, hence the awesomeness.

Jim is dead before the movie starts.
The movie opens with Jim, naked, in an abandoned hospital, having apparently woken from a coma caused by his getting run over by a car. This raises a few questions - why is he naked, and not in a hospital gown? Why was he seemingly the only patient left in the hospital? Why is his room so trashed? These points -- coupled with the surreal, hellish experiences he goes through before the ultimately hopeful ending and the slightly Jesus-like way in which he is framed in the opening shot -- make it look like Jim died when he was hit by the car. The entire movie is his soul going through torment in purgatory before finally being accepted into paradise.

That's right, folks - it's an actual Everyone Is Jesus in Purgatory story we got here...
 * It makes sense. Selena is essentially good but thinks only of herself, making her self-centered and proud; she later mellows and redeems herself. The guy who got bit back at Jim's house lied about it and promptly died for it; the balances of his good qualities versus his bad (hitting on Selena, for example) finally tipped in a 'downward' direction. Major West is well-intentioned but less than ethical. The soldiers are all desperate and frightened, but still crazy, base rapists. And so on. Jim's naked because he's played by Cillian Murphy. It's the scariest scene in the entire thing.
 * Dude. Your Mileage May Vary. This troper thought the Cillian Murphy nudity was a thing of beauty.
 * Furthermore, the other people he encounters could either be angels and demons sent to try and tempt him to go either up or down, or fellow souls who are hung by their sins and judged accordingly.
 * Souls in Purgatory are often depicted as being shown suffering the torments they experienced in life- the lustful buffeted by winds where in life they were swayed by their desires, and so on. So maybe it's a mix of both- the damned trying to drag him down and the few still-human survivors. (What's sad is, this totally makes sense now. :: facepalms :: )
 * The only problem with this is that in religions that believe in a Purgatory you can't "be pulled down", by being in Purgatory you've already proven you're not bad enough for Hell and you just have to continue trying to reach redemption. That being said those who died could have failed for the moment and "respawned" elsewhere.

Jim is crazy as a bag of hammers.
The zombies are real, but Jim is not a good guy. All the male competition for the few female survivors? Dead as hell.
 * That may be canon.
 * Jim killed all the other guys and hallucinated it was zombe-monsters or other crazy humans!

This movie and I Am Legend take place in the same universe
Scientist developed a cure for cancer. In the USA, they tested it on humans; in the UK, they tested it on monkeys first. The Dark Stalkers and the "Infected" are the same, only the American Infected developed an allergy to the sun due to its cure being tested on people with cancer. Jim wakes up at the same moment I Am Legend starts.
 * Guess the cure worked then.

Left 4 Dead takes place in the same universe as the 28 series
Self explanatory, really. The special infected and the change in infection times can be hand-waved by the virus adapting to the new environment of a fantasy-counterpart Pennsylvania. That new government health agency that you see on all the evacuation posters was formed after all the stuff that happened in England.

The Rage 'virus' isn't a virus at all
Rage is a curse carried by blood. It releases the negative energy in everything it touches. This explains how it works so damn fast.

A side effect of the Rage virus is increased production of blood.
Or alternately, the body expelling all its bodily fluid, not just plain blood and vomit. Private Mailer should not have that much blood in him.

Jim is a long-term guest at Arkham Asylum
He got dosed with his own fear toxin in Batman Begins. Everything that happens in Twenty Eight Days Later is what his mind is experiencing while he is having his drug-induced Freak-Out.
 * Twenty Eight Weeks Later is what happens when the rest of Gotham gets dosed, obviously.

Finland survives the virus.
The fighter pilot in the end speaks Finnish in the radio. This suggest the rescue operation is not an international effort, but Finland as a country manages to keep its government and infrastructure intact, while the rest of Europe is ravaged by the Rage virus.

Finland becomes the leading nation of the rebuilt EU, although graciously accepting American assistance with troublesome outer island regions like Britain.

The virus mutates before it gets to the United States...
Everyone there turns into a shufflin' zombie, though.
 * Now the movie and Music Video are united.

Sgt. Doyle was once called Clint Barton
Purely based on the fact that he is an expert marksman and sniper... who's actor became the World's Greatest Marksman

The infection takes longer to kick in than it appears.
The virus works by causing the body to overproduce aggression-related hormones. These hormones are present in the bodily fluids in sufficient quantities that contact with them can affect others. This is why people show symptoms so early. It's more like a drug taking effect.