Viral Transformation

""Hey, Doctor Luce -- this proteus nanovirus -- isn't it supposed to, you know, go out of control infecting and transforming everyone, and like that?" "Of course not. Not unless you design it to. Here's how.""

- GURPS: Bio-Tech

There are forces, be they biological, mystical or technological, that can infect and change a person into something-- else. These tend to be highly contagious, spreading easily via physical contact, bite, ritual, or other means. The changes are seldom subtle, and often quite a shock for the victim, who will likely wonder what they have become. On the plus side, the Viral Transformation is usually Cursed with Awesome, giving the "victim" Stock Superpowers and the ability to give them to others.

By now you're probably wondering what the downside is to this, and why this "disease" hasn't been shared among all of humanity. Well, it's usually because it makes the transformees jerks. While it's not (always) The Virus, making those infected into Always Chaotic Evil, Mind Controlled servants to a Hive Queen who gladly eats puppies, it does lend itself to "Power Corrupts". Though the changes are mostly physical, the alterations may cause a change in perception because The Mind Is a Plaything of the Body, their new body has a Horror Hunger only sated by eating orphans (and it tastes good!), and With Great Power Comes Great Insanity.

Those responsible for infecting the victim may expect a Face Heel Turn because the shock at the change will make them run to others like them, as well as Fantastic Racism making it likely they'll be alienated by their friends and family (this is especially likely if the transformation is less than cosmetically appealing). However; the choice to do so remains with the victim: they can consciously choose Transhuman Treachery, to stand apart and above of humanity, become Fully Embraced Fiends who enjoy their condition, to be Friendly Neighborhood Vampires and live among if not entirely in harmony with humanity, or even be a Pro-Human Transhuman and act the Sheep in Wolf's Clothing while becoming a Vampire Hunter.

Common examples of this include most forms of Lycanthropy and some forms of Vampirism. Expect those affected by a Viral Transformation to be tempted to help friends or loved ones with an Emergency Transformation or surprised when a blood donation turns into a Superhuman Transfusion. See also Virus Victim Symptoms.

Not a transformation into that guy named Viral. See also Warm Bloodbags Are Everywhere. Compare The Corruption. A type of Face Monster Turn.

Anime & Manga

 * Blassreiter. After being infected with body-modification nanobots people become Made of Iron and can meld with various hardware, but generally also turn into omnicidal maniacs with hallucinations and/or damaged memory.
 * Used in the Manga series Dragon Eye, when the villains are created when the D-Virus changes them into a monster known as a Dracule. Anyone infected loses their identity and goes on a killing spree. Only a few, (about one in a hundred/thousand) have the antibody resistance that changes chance of infection from 100% to 10-0.1%.
 * The Eclipse infection in Magical Record Lyrical Nanoha Force. The good news is that, when infected, you gain powerful magic, Healing Factor, Anti-Magic, inexhaustible endurance, and a whole buttload of other stuff. The bad news is that, if you want to keep your life and sanity, you need to keep on killing to keep the infection in check.
 * Becoming an Homunculus in the world of Busou Renkin sounds great - increased strength, a range of superpowers, Immortality, immunity to disease... but the fine print is that your diet now consists of people. And if you're not a super-special-awesome Human-type Homunculus, you go insane and become dominated by your instincts.
 * Scary Monsters, the Stand-ability of Doctor Ferdinand from Jo Jos Bizarre Adventure: Steel Ball Run infects any living creature with a virus that turns them into a dinosaur under Ferdinand's control. Then the virus is spread by the dinosaurs themselves to practically create a zombie infestation... of dinosaurs. Diego Brando is later able to keep and control the infection on him due to obtaining the Saint's eye but Diego's version seems to lack the infecting properties.

Comic Books

 * The Brood from Marvel comics propagate their species this way. They inject parasites into viable hosts (which is basically everyone) that gradually transform the hosts into more Brood. By the time the transformation's complete, the original psyche of the host is all but gone. Incredibly, despite being (nearly) Always Chaotic Evil, it's still possible to pity the Brood because this is the only way they can reproduce.
 * Druuna comics have diseases that change people into tentacled monsters.
 * The "Monstar Virus", pretty much Exactly What It Says on the Tin, in the Amory Wars.

Literature

 * This is the basis for the Wild Cards series of books. An alien virus gives some people super-powers and transforms others into "Jokers", who are sometimes not even recognizable as having been human.
 * Well, at least if your particular draw included the ability to infect others. Despite the fact that xenovirus takis-A was a literal virus, it wasn't (normally) communicable.
 * In Neverwhere, spending too much time with people from London Below -- or if they "go too fast" around a normal-- will make the person go out of sync with people in London Above, making them Invisible to Normals.
 * In Jeff VanderMeer's Shriek: An Afterword a fringe historian Duncan Shriek is the only human to discover a safe method to enter the underground realm of the Graycaps under the city of Ambergris, but this involved getting an esoteric fungal infection, which would in the course of the years transform him into a man-fungus hybrid who could only retain human shape through constant mental effort.
 * The novel also features common people getting less extreme, though still bizzare-looking fungus infections out of the fungus bombs used during the civil war. And the sequel, Finch, has Partials, people who voluntarily go through this process to become akin to fungus cyborgs.
 * In The Relic by Douglas Preston and Lincoln Child,  by inserting saurian and reptilian DNA into host cells; but the victim needs a steady supply of specialized hormones to  ; in order to acquire these hormones, the victim must go right to the richest source available and eat the hypothalamus of its victims or go mad from the pain of being unable to sustain.
 * It's very common in zombie novels for the zombification to be caused by a virus, the most famous or at least one of the earliest probably being I Am Legend, both the book and movies.

Film

 * Creepshow segment "The Lonesome Death of Jordy Verrill". After the title character is infected with liquid from a meteorite, he slowly changes into a fungus creature. He manages to end his suffering by committing suicide with a shotgun. The infection is later shown spreading across the countryside, creating a beautiful verdant landscape.
 * The plot was obviously copied from H. P. Lovecraft's The Colour Out of Space.
 * Underworld has examples of both this and The Virus. The original flavor werewolves under William were all raving savages, but those who inherited it from Lucian spend most of their time in human form, and can eventually enter or leave were-form at will.
 * Then you have the various takes on the hybrids. The movies show two different hybrids (Marcus and Michael) and mention a third (Lucian and Sonya's unborn child). The first two involve a member of one species getting exposed to the blood (and virus) of the other. While the viruses normally annihilate each other, the presence of the "Corvinus strain" in both allowed the viruses to merge into something new. In Michael's case, his original Lycan strain retained dominance, making him appear vastly more human than an average Lycan. For Marcus, who started as a vampire, attained more bat-like characteristics, although he did gain several Lycan ones as well. It is unknown what Lucian and Sonya's child would look like, given that it was equal parts Lycan and vampire, or if it would survive at all, given the mutually-destructive "relationship" the two virus strains have.
 * In District 9 this happens

Live Action Television

 * Doctor Who; The Empty Child/The Doctor Dances has people being transformed into zombie-like beings with gas-masks fixed on their heads. The cause are
 * The Waters of Mars sees people touched with infected water turned into zombie like creatures that spray more infected water to spread the virus further.
 * And the cliffhanger for The End Of Time part 1 has
 * In the classic series, we have the Krynoids. A Krynoid is a monstrous plant that  Two men were converted in the Krynoids' debut story, The Seeds of Doom  . This so outraged Mary Whitehouse, a media watchdog, that she spoke out about the "obscene vegetable matter" being shown on Doctor Who, which she dubbed "teatime brutality for tots".
 * The Stargate Verse loves this trope.
 * Stargate SG-1: A virus caused people to turn into cavemen (a minor version, it just gave them funny foreheads and heightened aggression). A nanotech version made O'Neill 100 years old in a few days. Teal'c was bitten by a bug whose venom would have converted his total body mass into more bugs.
 * Stargate Atlantis: The Iratus bug retrovirus designed to "cure" Wraith does this. First it turned a female Wraith and then Sheppard into human/bug hybrids. Once perfected, it turned Wraith into humans. Michael, one such victim, took it and used it to make Wraith/human hybrids.
 * Stargate Universe:.
 * Towards the end of Maddigan's Quest, it's revealed that
 * Ilyria was played as a variant of this on Angel, infecting Fred after she inhaled dust from Illyria's sarcophagus and then turning her slowly into the shell.

Tabletop Games

 * Cybergeneration, a sequel to Cyberpunk, is run on the premise that a nanite infection will kill anyone over the age of 16. If you're under 16 you survive... or change.

Video Games

 * Members of the Many in System Shock 2 undergo this, tending to leave behind a lot of Apocalyptic Logs. While some engage in Transhuman Treachery, a large amount of the Nightmare Fuel is delivered by the poor Red Shirts who beg you to flee while they're unwillingly chasing you.
 * In Lost Souls MUD, a local implementation of Nethack's "eating a mind flayer makes you smarter" makes it so that eating the neural tissue of the vlekthid (read illithid) or gogtzul (read Star-Spawn of Cthulhu) races results in that neural tissue colonizing your own nervous system, making you smarter and less emotional, and giving you both psionic powers and often-crippling mental disorders.
 * Borderlands' first expansion, the horror movie themed The Zombie Island of Dr. Z Ned, has both zombies and Wereskags (skags being the planet's equivalent to dogs/wolves). Both transmit their infection/curse via bites.
 * Albert Wesker of Resident Evil is infected with such a virus. It is not particularly transmissible, but it does make him something of an unstoppable badass.
 * Redlight and Blacklight from Prototype. Redlight is a completely straight version of The Virus, which transforms people into braindead, malformed husks serving Elizabeth Greene, while Blacklight is a Viral Transformation.
 * Assuming that you haven't been eating all the civilians, anyway.
 * Everyone is infected in Left 4 Dead. If you're lucky, nothing about you changes and you become a carrier/survivor. If you're unlucky, you become a common infected. If you're EXTREMELY unlucky, you become a special infected.
 * Mitochondria Eve of Parasite Eve fame is basically a sentient and controlled version of this by manipulating the mitochondria cells in any biological lifeform, whether it be man or beast. The beasts get some pretty wicked transformations. The humans however...not so much
 * Disgaea 4: A Promise Unforgotten has the A Virus, which slowly transforms anyone it affects into.

Web Original

 * A Shared Universe called The Blind Pig - and its reboot/successor Pig and Whistle - take place after a virus with unknown origins spreads throughout the Earth. A lot of the people who get it die. Some percentage of the survivors transform. A smaller percent of them get unusual abilities.
 * The Paradise setting involves a "reality virus" that transforms humans into Funny Animals at a rate that doubles every year.

Western Animation

 * In Generator Rex when someone's (or something's) nanites activate they almost certainly become mindless killing monsters (called E.V.O.s) that try to attack anything and everything. You can also expect said monsters to be nightmarish and have a good bit of Body Horror thrown in. And the rare E.V.O.s that retain their mind/personality/human appearence are people who you probably don't want to hang around with (except Rex). There are also those that turn into freaks but retain their personality, but they tend to either become victims of Fantastic Racism or also be people you'd rather not be around.
 * And said nanites that can mutate anything at any given time? They're omni-present and only the titular character can effectively cure aforementioned E.V.O.s.

Web Comics

 * In El Goonish Shive, whoever touches the Dewitchery Diamond while under a "curse" (for a very loose definition of a curse) will be split into their normal state and a duplicate who not only is the embodiment of the "curse" as applied to the original person but who can spread it to others no matter what the nature of the curse is. Fortunately, if the "curse" is easily reversed, the people the duplicate transforms can have the transformation reversed just as easily.
 * It's not exactly clear where all the "devils" in Sinfest came from, but at least some of them were originally human, and one of those has shown the ability to transform animals into their demonic equivalents. (Similarly, holy power can turn a devil human. The strip's version of Jesus spends a lot of time undoing transformations that its version of Satan has inflicted.)