Galaxy Express 999's Machine Empire is an intriguing subversion, for while they are rather obviously cut from the same cloth as the Cybermen and the Borg, and wage brutal wars of conquest, they don't generally convert conquered races by force. In a late episode featuring a planet whose government, a minor satrap of the Empire, had recently begun forcibly converting humans, a leader of the Empire actually rebukes a local official for the damage this policy was doing to their cause.
This happens to entire planets in the Salamander OVA.
Higurashi no Naku Koro ni takes this in a somewhat weirdly literal direction with Hinamizawa Syndrome. For the vast majority of the infected the virus stays dormant for their entire lives, but once someone's case has been aggravated, you really can't bring them back easily. Although there are exceptions.
Blue Drop has a somewhat lesser example. The second manga, Tenshi no Bokura, implies that any human female sexually exposed to the One-Gender Race Arume (a species of alien lesbians who reproduce by a complex-and-imperfect genetically engineered Homosexual Reproduction process) becomes somehow psychologically incapable of loving a man, possibly due to special pheremones (a form of chemical warfare?). The implications for both species, as well as the results for society, are truly horrific.
The anime of the series Le Chevalier d'Eon has this trope. In that people being controlled by the Psalms turn into "ghouls" who bleed silvery blood and their flesh is all gross looking. They don't seem to have control over their bodies, though in typical fashion the "infected" may have some dying words such as "kill me" to mutter.
The series Red Garden has this as well. In the first episodes those affected by "the virus" appear to be superhuman. One of them also walks like a dog. Consequently, it ends up being a curse. Two rival clans, Dolores and Animus, each stole from the other a Book Of Curses and used it against the other. The protagonists' boss works for the Animus clan, who cursed people of Dolores descent to turn into mindless and feral killers. The way it manifests still acts like this trope, however.
The Kanshuu from World Embryo. Traveling through cell phone signals, any person who listens to their phones and hears these signals mutates and contorts into another Kanshu. It gets worse since anyone who knew those victims in life, be it parents, friends, siblings, etc., will have their existence completely wiped from their memories, causing them horrible mental trauma.
The OMAC from The DCU are complex nanomachines that hide in a person's body until they are kickstarted through a command by Brother I. They proceed to take over the inhabitant's body and turn him into one of the electric blue, one-eyed killing machines. Ironically, they were spread through tainted flu shots. It's an awfully sexist virus, too, as it will seemingly convert even a female human into a male OMAC!
For a Marvel Universe equivalent, during the "Operation Zero Tolerance" arc, Bastion turned certain people into "Omega Sentinels" by abducting them, filling them with nano- and cybernetics, and placing a sleeper program in them that would activate in the presence of mutants.
The "Legion of the Damned" arc in DC's post-Zero HourLegion of Super-Heroes features the Blight, which ravages the United Planets and takes over most of the Legion before the remaining Legionnaires manage to purge it.
In DCU's Final Crisis, Big Bad Darkseid uses the Anti-Life Equation this way, bringing Earth under his control, and transforming some of the heroes into his monstrous servants. Ironically, Checkmate uses the millions of people infected with OMAC nanotechnology left over from the last crisis in order to fight against the Anti-Life Justifiers, making this a Virus versus Virus situation.
This was the power of Weapon XII, a villain appearing in New X-Men #129-130. Every person or animal he touches develops glowing eyes and permanently becomes an extension of his mind. By the time Xavier and Jean Grey arrive to deal with him, he's infected most of the humans in the Chunnel, several hundred of Multiple Man's bodies, and a collection of birds, bats, and dogs.
The Marvel Zombies series features a zombie virus that infects superhuman beings, who then devour the rest of the population, until their hunger leads them to travel to other planets and eventually other universes, seeking more food and infecting more superbeings in the process.
In Project Tatterdemalion, a Sci Fi/horror AU of Bleach, Hollows are the result of an alien Synthetic Plague called "Madsen's Hollow". It transforms one-fifth of the infectees- those with all of a set of genes that make them vulnerable to it- into monsters with Combat Tentacles and a drive to infect others to reduce their loneliness. The other eighty percent die messily from the incomplete effect. Interestingly, there is a vaccine- although it does have some side effects.
Film - Animation
Wreck-It Ralph plays with this with the Cy-Bugs, which assimilate anything they eat, including the main antagonist of the story.
The 2009 film Daybreakers plays with this in interesting ways. Humans bitten by vampires will turn into vampires, resulting in almost the entire population being turned by the time the film is set. However it turns out that drinking the blood of a vampire-turned-human is a cure. Near the end of the film, everyone is suffering from deprivation as a result of the dwindling human population, so several soldiers mindlessly feed on one of the former vamps. When they turn back, they're then fed on by their starving comrades who also turn back, rippling outward until there are a few survivors left in the room.
Agent Smith in The Matrix sequels. After Neo tried to delete him, he comes back and now can copy himself into other people. Ironic, given that he was disgusted by the human race and called them a virus in the first movie. Also, for symbolism points, his sunglasses changed after his resurrection. Originally they were square, and now they're slightly more rounded, and have the general shape of a protein capsule of most viruses.
Throughout the Matrix Trilogy all the Zionites and their supporters (Seraph, The Oracle) have round glasses while Agents and their sympathizers (Cypher) wear square ones. The fact that Smith's glasses are something in between furthers the symbolism and is another example of the series' Fridge Brilliance.
The 2008 horror movie Quarantine (and the movie it was a remake of, REC) picked up on the appropriateness of rabies as an example of The Virus by making a mutated version the film's threat. In the original REC it was suggested that The Virus has a more... supernatural origin. Which the sequel shows as Demonic Possession.
As the name implies, the antagonist from the 1999 film Virus. A transmission from space takes control of a (seagoing) ship's computers and begins building something. When the heroes ask the program what it wants, it replies with a list of body parts.
The alien in Slither, which spreads through parasites that turn hosts into drones for the Hive Queen, controlled through a Hive Mind.
Vampires. In almost all versions of those myths.
Werewolves in more modern versions.
Zombies in some more modern versions.
That most dreaded and contagious of schoolyard infections: Cooties!
The Yeerks from K. A. Applegate's book series Animorphs. They are a race of parasitic slugs who infest a host body by entering through the ear and wrapping themselves around its brain. It can then read the hosts mind and control the host's every action, imitating its behavior. The mind is left a prisoner, able to think freely but unable to control any aspect of their physical selves. They are returned to themselves briefly every three days, when the Yeerk needs to feed.
A unique example from Charles Stross' SF novel Singularity Sky: Mimes. Mimes who reproduce themselves by hitting humans in the face with pies — the pies are full of nanotech that reformats the human into a new Mime, complete with more pies. Lucky for those being pursued by Mimes, they are afflicted with the sort of tics you'd expect, such as occasionally being trapped inside an invisible box or having to walk against the wind.
Stross also created "Curious Yellow" in the novel Glasshouse. CY was a digital virus that was capable of controlling people and removed their memories (as well as computer records) of... something. What no one ever found out, even after the long, brutal war to contain and defeat The Virus.
A particularly unusual example shows up in Philip K Dick's novel The Three Stigmata of Palmer Eldritch. The title character distributes an exceptionally trippy drug with the side effect of causing the users to think more like he does than they used to — and also causes them to spontaneously develop Palmer's "three stigmata", which are artificial eyes, a metal jaw, and a replacement arm.
One of the creepiest examples ever is Philip K. Dick's short story "Upon the Dull Earth". A weird necrophiliac woman attempts a bizarre experiment to speak to the "angels" who supernaturally rule the Earth, which fails and claims her life. Her grieving boyfriend bargains with the angels to bring her back in a Deal with the Devil. Unfortunately the angels screw up when doing so — the girl comes back, but only by hijacking another person's body, taking over her sister's body and physically transforming it into her own. The angels seem unable to stop this process, either — soon everyone in the world begins spontaneously transforming into a duplicate of the girl. Madness and horror ensues.
Another squicky example occurs in Spiral, the first sequel to Ring. Any time a woman watches the cursed videotape (or any of its derivatives), whilst ovulating, they become pregnant with a clone of Sadako, which develops to birth in a week, leaving the host to die.
The Solanum virus from the Max Brooks book World War Z. Lethally toxic to all animal life if ingested. Kills and reanimates humans bitten or scratched by the infected or exposed to their bodily fluids, and can potentiality reanimate infected people who die from other causes while the virus is incubating. Does NOT raise long-dead people from their graves.
The Conjoiners, a Hive Mind culture in Alastair Reynolds's Revelation Space works, are actually the good guys in a number of stories. They started out as a handful of scientists who linked their minds as an experiment and subsequently released a virus to assimilate more minds into the collective, but later their goal became simply to survive. They consider conscripting their enemies a more humane alternative to killing them.
In Steven Hall's The Raw Shark Texts, Victorian "gentleman scientist" Mycroft Ward gradually becomes this. He cheats death by transferring his personality into a younger man, and this new body cheats it again by sharing knowledge with a second person. It all goes bad when the increased self-preservation instinct causes him to search out more and more hosts.
Doctor Who: Faction Paradox in the novels had "Faction biodata", which they could inject into such things as regenerating Time Lords or pure life energy creatures melding with a planet's ecosystem, turning their hapless victim into a tool of the Faction via an extremely vicious form of Cosmic Retcon. This would normally be bad. It becomes horrifically bad when the infected Time Lord in Interference is the Doctor.
Vord in Jim Butcher's Codex Alera are a sort of gumbo version of this, combining several Virus tropes into one vicious nasty.
Dances of the Men by V. Pokrovsky has a new one, in case old bites and microbes were too easy to block. One guy invented a Super Serum-level brain stimulation by complex microwave pulses. When he tried this trick on H.Sapiens, human brain turned out to be different—it re-emitted this. With the power growing until it can melt holes in wire veils at point-blank range, just before boiling itself. Not that it mattered to that veil's owner much—the powers it still gave are deadlier anyway, and most of infectees at the final stage are violently insane. Victims just infected through the wall at the other end of the street emit less than noise level until they get worse, so Im[pulse]Patho[logy] kept to re-emerge.
In The Wheel of Time, 13channelers weaving a particular kind of Black Magic through 13 Myrddraal can forcibly turn any other channeler to the Dark Side. The victims are described as having unnatural facial expressions, "like the smile on the lips of a corpse", and "something not-quite-alive inside those eyes. This didn't seem to be a man, but a parody of one. A shadow stuffed inside human skin."
In the Star Wars Expanded Universe novels Death Troopers and Red Harvest a virus is delivered through a chemical compound derived from a plant and other Sith formulas, which turns the victim into Zombies. Not kidding, ZOMBIES in Star Wars. The virus is transmitted through bites or exposure to infected blood, and the horde even some kind of hive mind to boot.
In Neil Lee Thompsett's Becoming Human, humanity itself is this. Not a species, a disease.
In Tom Kratman's Caliphate, the three renegade US scientists led by Claude Meara made a flu virus called H5NV, designed to be 97% fatal, with the remaining 3% expected to become significantly harmed even if they survive.
Legacy of the Dragokin: Jihadain's monsters can transform their victim into the same kind of monster by biting them. In this way she create an army from muggles.
In Wen Spencer's Ukiah Oregon books, the villains are this. Oddly, so are the heroes.
Buffy vampires have their previous selves' souls replaced with a demon (though it can be retrieved later by a spell). This is actually a reversion to the original vampire legends in Eastern Europe, in which spirits called upir, which are supposed to be simple psychopomps (escorters of the dead to the afterlife), glom into the now-soulless bodies and walk about in search of fun. However, despite not really being the same entity, the vampires retain their memories and usually end up being a twist on the original person's personality, particularly the repressed parts:
Willow: It's horrible! That's me as a vampire? I'm so evil and... skanky. And I think I'm kinda gay. Buffy: Willow, just remember, a vampire's personality has nothing to do with the person it was. Angel: Well, actually... (sees Buffy's expression) ... That's a good point.
Angel's response above reveals that the common explanation for vampirism doesn't quite work for all vampires. In Angel when Angel's soul is again lost and the gang attempt to put it back, Angel has the chance to actually interact with his 'alter-ego' Angelus via a drug-induced dreamstate. Angelus puts it to Angel that they are simply two sides of the same person rather than literally being two distinct personalities, since Angel is perfectly capable of exactly the same kinds of vicious acts. 'Angel' just regrets it afterward.
In the episode "Bad Eggs", students participating in a health project raise eggs as if they were children. The eggs end up hatching and the demons inside of them take control of whoever was supposed to take care of them. All of the egg-demons are controlled by the "mother" demon below the school.
Werewolves are created when a person is bitten by another person and survives. The interesting bit is that said werewolf doesn't have to be transformed when the bite occurs.
Jasmine from Season 4 of Angel turns everyone into her fanatical worshipers willing to kill others for trying to free them from her influence. Ironically though, being infected with her blood frees you from her control. Jasmine refers to it as "spreading the hate".
The Yeerks from Animorphs. They are a race of parasitic slugs who infest a host body by entering through the ear and wrapping themselves around its brain. It can then read the hosts mind and control the host's every action, imitating its behavior. The mind is left a prisoner, able to think freely but unable to control any aspect of their physical selves. They are returned to themselves briefly every three days, when the Yeerk needs to feed.
The Cybermen of Doctor Who, who were arguably the inspiration for the Borg.
The "Empty Child" in the new Doctor Who series (from the episodes "The Empty Child" and "The Doctor Dances") was a little boy raised from the dead by alien medical nanomachines that had no clue how to rebuild humans. Anytime he touched someone, the nanomachines would reprogram them to the way they thought all humans should be — down to the gasmask and injuries the boy had when they found him.
And the Primords in "Inferno". If they even touch you — hell, if you touch that green stuff for even a fraction of a second — it's only a matter of time before you become a mindless Primord.
The Flood from "The Waters of Mars", literally a (water-borne) virus. An unusual one, as it can exist and be dangerous outside of a host as well. It's harder to avoid and does its work faster than most of the examples on this page.
Almost every new season/series has at least one. The diseased people in "New Earth" try to get others sick, although it's not clear that a "second-generation" infectee would do the same. "Tooth and Claw" has an actual werewolf. A temperature-raising infection spreads in "42". "The Family of Blood" works like this. "Midnight" involves the passengers thinking this is going on. "The Time of Angels" adds something like this to the abilities of the Weeping Angels. "The Vampires of Venice" naturally has vampires. And "Night Terrors" combines this with Creepy Doll.
Supernatural: The demonic virus from the episode "Croatoan". No explanation was given (at the time) why it didn't affect Sam but affected everyone else who was exposed to it.
The revelation at the end of season two that Azazel infected Sam with his own blood the night of the fire more than likely covers it. The virus comes back in season 5 when Dean travels 5 years into the future (2014) after the start of The Apocalypse.
"The Sickness" from LOST is almost certainly this. For five seasons it was suggested that the Sickness was just the delusion of a madwoman... until season 6, when Claire and Sayid become infected, start following the orders of the Big Bad and kill a whole bunch of people.
On Lexx, the Big Bad of Season 2 is Mantrid, a Mad Scientist in a robotic body who creates an army of flying robotic arms that convert other living beings into more flying robotic arms. Eventually, every living thing in the Light Universe except for the Lexx and its crew is converted into Mantrid's army. Not just every living thing. Every thing. Including the stars.
Epideme from Red Dwarf is an intelligent artificial virus designed to block nicotine cravings. Unfortunately, he decided he'd rather spend his time stealing his various hosts' knowledge before killing them by starving their bodies of nutrients and oxygen, then hijacking their bodies, each time forcing them to bite someone else and transfer him to a new host.
In Community episode "Epidemiology" Greendale becomes the epicenter of the outbreak for an experimental virus designed by the army.
Episode 5 of Danger 5 features a contagion that's turning the Allied soldiers into Those Wacky Nazis. Its source is a literal wellspring of the "Aryan seed" and it is sexually transmitted. Exposing it to Swiss blood is the antidote, since in the Dangerverse, Swiss blood is... made of gold...
JAG: Sudanese Ambassador Moshak in "Embassy" stole vials of Ebola from a U.S. military lab, and when Harm is about to get him, he throws a vial of Ebola at him. Fortunately Harm caught it.
Teen Wolf: Seems to be the case with lycanthropy, but mainly to those who have been turned and never wanted to be a werewolf to begin with. Some people, like Derek and other members of the Hale family, are naturally born with it.
Dungeons & Dragons has the olive slime, yellow musk creeper, and other monsters of this kind:
One prominent example is the vargouille; it's a relatively low-level monster (Challenge Rating 2) that reproduces by "kissing" members of another species. Unless this kiss is cured with an appropriate spell, over 24 hours, the victim's hair falls out, their head becomes more wrinkly, the ears grow and become wings, and eventually, the head separates from the body and becomes another vargouille. It was even more creepy in the first edition, when the separated head would still have a variety of internal organs attached, dangling down from the neck.
Many undead have the power to turn anyone they kill or Level Drain to death into an undead of the same kind under the control of their killer.
Mind Seed is one of the most insidious psionic powers in 3.5. Implant someone with a Mind Seed and it will slowly reshape the victim's mind- against their will or even without them noticing- into a mental replica of the psion at the time the power was used. Though the resulting personality will still have their own free will, the fact that it now shares the goals of the psion makes it incredibly effective.
Powers of Chaos, which can quite effectively turn the open mind into a willing cultist and a traitor to mankind.
Tyranidswarms, the insidious Genestealer cults, the insidious Tau propagandists/brainwashers, the insidious NecronPariah harvest, etc., etc., etc. Rarely is any given conflict zone in 40K NOT subject to some form of The Virus. Let's just say that in this universe, General Ripper typically has the right idea.
The dreaded Obliterator virus, which turns you into something truly horrific. The first tip that something horrible is going on is when you realise that you're spontaneously generating ammunition for your gun, which is becoming gradually more attached to your hand. From here, it's only a short trip to the point where you're an out-and-out psycho who can absorb guns, then create them again fused to your flesh. When compared to the fates of some Chaos cultists, especially those who fail (think living flesh bag of pain and get worse), the bloodthirsty insanity and difficulty of speech is not that bad and the Body Horror comparatively mild. Once transformed, you're likely to see your fate as Cursed with Awesome.
Another electronic example is The Virus in Traveller: The New Era, originally designed as a weapon for shutting down the navigation systems of enemy warships in order to end the war without further bloodshed. Unfortunately the version that was prematurely unleashed when the research station working on it was attacked would shut down every computer in the vicinity through means unexplained (though heavily implied to be psionic in nature), with... interesting consequences if that computer happened to control (for instance) a nuclear reactor or a life-support system. Then it evolved full intelligence, which didn't improve matters.
One of Feng Shui's many Creature Powers is "Corruption", which allows a supernatural creature to infect others with their supernatural essence, and in this way create more of their kind. Corpse factories in the Glimpse of the Abyss supplement use a variant of this to create zombies for all your Zombie Apocalypse needs.
The short-lived Nightlife horror RPG had a race of borg-like monsters that embodied The Virus trope. Surprise, surprise, they were called "the Virus".
The premise behind the Cyberpunk sequel game, Cybergeneration.
Phyrexia, in all its incarnations, has this as one of its most prominent themes. Several New Phyrexian cards are older cards given a Phyrexian twist.
A legendary creature card from the Innistrad set, Olivia Voldaren, is a classic infectious vampire: one of her abilities deals a point of damage to a creature and turns it into a vampire, and her other ability allows her controller to take control of any vampire as long as she remains in play.
The Exsurgent virus family in Eclipse Phase. Different strains can infect computers...or flesh...and the result is never, ever pretty, usually featuring Body Horror and always featuring Mind Rape. Oh, and they're very, very adaptive. Like, normal-viruses-on-crack adaptive.
"What’s worse to contemplate, though, is that we may get another major outbreak that spreads to multiple habitats before we can contain it. That might get very, very bad, very, very quickly."
A third-party Pathfinder supplement inverts this with the Obitu race, a race of living skeletons created by a virus that only infects the undead.
Nobilis: Actuals that are drawn out of the spirit world tend to occupy people or things to this kind of effect. One sample Actual, the Family, consists of an entire clan of elegant, long-haired and innocent people with eyes the colour of pale quartz, who take in people in need and help them, only for those people to eventually transform into more members of the Family.
The opera Help, Help, The Globolinks! has invading aliens called Globolinks whose only known weakness is music. Humans touched by Globolinks are gradually transformed into Globolinks, first losing the ability to speak human language.
In Eugene Ionesco's absurdist play Rhinoceros, the townsfolk are all spontaneously transforming into rhinos.
The Makuta's virus and the Dreaming plague in BIONICLE, although it's ambiguous as to whether the latter was actually transmitted virally or not.
The character Alex Merceris The Virus. Under the right circumstances, The Virus gives a human being the powers to fly around and kill stuff with viral blades and whips and shit (as in the case of the main character); otherwise, you'll turn into a mindless, shambling, living zombie or a mutant with augmented speed and strength (as in the case of a lot of New Yorkers). It also infests warehouses (and ONLY warehouses for some obscure reason) to produce beefed up mutants that are incredibly annoying.
The Big BadElizabeth Greene is the other virus, and because of her aggressive nature and the fact that Mercer's virus seems inert (read: non-transferable) in him after the initial infection at Penn Station, she's more dangerous anyway. Alex can't get half the bodycount she does unless the player's doing it on purpose. The viral Alex, anyway.
Crysis 2 has a viral outbreak in Manhattan that may or may not be from the Ceph, and certainly wasn't mentioned at all in the original Crysis. A group called C.E.L.L. apparently is tasked with containing it at any cost...including the murder of any potential carriers.
Bacterion, Gofer, and Zelos of the Gradius series are incarnations of The Virus; Every time you defeat any one of these guys, the Bacterian cells that they are composed of will increase in numbers. Dr. Venom also applies to this too, since he's been modified by the Bacterians.
In Resistance: Fall of Man, an alternate history first-person shooter, the aliens known as the Chimera use a mutagenic virus that causes humans to fall into a coma and undergo horrific physical mutations that turn them into Chimera soldiers. These mutated soldiers retain no trace of their former personality and are completely subservient to the elite Chimera known as Angels; in fact, Chimera soldiers die if their psychic link with the Angels is severed. They needed a Conversion Center to fully complete the transformation in a reasonable amount of time. In R2, infected people simply have cocoons built around them and are left to transform on their own.
Appears in the WarCraft series. The Lich King produces a virus which kills humans, then resurrects them as undead loyal to him. There is also a faction of Undead, the "Forsaken", who have broken free of the Lich King's control and have regained their free will.
In World of Warcraft, the Old Gods' Curse of Flesh "corrupted" the creations of the Titans on Azeroth into fleshy beings like themselves. In other words, every human, dwarf, gnome, and trogg (and possibly a few other species too) is "infected" by The Virus!
The Flood in the Halo franchise, which latches onto sentient life forms, hijacks their body, mutates them horribly, and, when the host is no longer usable, uses it to incubate more Flood infection forms. The infection forms even look like macroscopic bacteriophages. When the Flood hijack enough bodies, they build a Gravemind; a giant, rhyming, Magnificent BastardHive Mind. To make it even worse, the minds and memories are assimilated, and it's possible that the living mind of the victim gets assimilated (in backstory said MagnificentBastard has a habbit of bragging about transhumanism and ending suffering). This allows the Flood to, after a certain stage of outbreak, use any tool you can, if they can get their grubby tentacles onto it. Coupled with icky Body Horror and the assimilation and hacking of even spaceships, this Nightmare Fuel isn't too much of a stretch to imagine wiping out an entire pan-galactic civilization... which they are very heavily implied to have done before they arrived in the Milky Way galaxy and encountered the Forerunners.
The Forerunner Saga reveals that they're actually the malevolent remains of a highly-advanced species known only as "the Precursors"; during their conflict against the Forerunners, they literally crushed entire planets with Precursor technology and could corrupt AIs with telepathy alone.
Though it involves no physical changes aside from a possible palette swap, the Maverick/Zero Virus from the Mega Man X franchise otherwise qualifies.
After the Maverick Virus, Mega Man ZX has Model W gained the ability to manipulate the thoughts of humans and Reploids alike. This became important for Albert's Evil Plan.
The Roboenza in Mega Man 10 is essentially a robotic equivalent of the flu. Except for the fact that it causes infected robots to go berserk for no real reason. It was made by Dr. Wily of course, despite his claims to the contrary.
Left 4 Dead with the infection. In most cases the virus will turn most people into unintelligent "runner" zombies who will psychotically attack any uninfected individual. Body Horror comes into play when some of the infected mutate into special infected that feature warped bodies optimized for specific special abilities. The virus spreads by the infected transmitting bodily fluids (I.E. Getting bitten or being exposed to zombie bodily fluid), but it can also be spread by unknown means by Carriers, individuals who show no signs of infection but can spread the infection.
Sacrifice comic reveals that the virus mutates daily. One day it's airborne, nex day it's something else.
The Tuurngait virus in the Penumbra series: Able to infect pretty much everything. Causes the infected to become stronger and more resilient or just ridiculously huge, depending on species. The virus is also sentient and controls all of the infected through telepathy. Oh, and it is older than humanity.
They started up a fairly innocuous power-boost, except in a few very specially treated individuals, who began to suffer from mad wanderlust and lose their minds. They then attempt to join up with the head of Jenova. Naturally, the main character, who was traveling all over the world for a reason he could hardly justify, based on little more than instinct, and seemed to suffer occasional but horrific mental episodes, turned out to be one himself.
Jenova's machinations go much further than this. According to Ifalna, in her interviews with Professor Gast, Jenova first landed on the Planet and began spreading its cells like a virus, mutating the Cetra into horrible monsters under its control. The monsters employed the same abilities as mommy, able to absorb the appearance and memories of their hosts, and quickly slaughtered a whole civilization by imitating their loved ones. It seems that Jenova itself is an entity that travels from Planet to Planet, infecting the native inhabitants and transforming them into carriers to further spread its infection around, before bringing them all back and "reuniting" itself to travel to another Planet and continue the cycle. How long it's been doing this is anyone's guess.
The Super Mutants from the Fallout games were created by the Forced Evolutionary Virus; the first game culminated in the player destroying the transformation vats, although they could agree to be turned into a Super Mutant to achieve a Non Standard Game Over.
The plot of the Fallout 3 expansion pack The Pitt centers around the Troglodyte Degeneration Contagion. Most victims only suffer skin lesions, but the more severely affected lose their minds and become Wildmen, or worse, fully degenerate into Trogs. Nearly all children born in infested area become Trogs within a few weeks of birth, with the exception of Marie Ashur, the daughter of the slavers' leader, who has an immunity to TDC, offering hope for an eventual cure.
Vault 22 in Fallout: New Vegas is infected by a parasitic fungus that mutates exposed humans into Spore Carriers, which spread the fungus further; some of the fungus has escaped as far as Zion Canyon and Big Mountain, as evident by the presence of Carriers there.
Metroid Prime's Phazon is an example, but to very varying degrees, depending on the game and target.
In Prime 2, the Ing also have a tendency to take over the bodies of other creatures, both living and dead.
The X Parasites from Metroid Fusion are a more traditional version of this trope, as their only purpose is to infect more people.
Mass Effect 1 includes the Thorian, which is a plant-like entity that uses spores to mentally control people through pain. Also, there's Sovereign, a Reaper which controls the minds of its victims through a process known as "indoctrination". People controlled by Sovereign can only be saved by killing them. Both cases also involve instances where people choose to overcome the mind-control the only way they can.
To further fit this trope, unguided or faulty indoctrination seems to wake the desire to become a cyberzombie, leading the victims to jump onto the zombiefying devices. This may or may not be intended as a failsafe.
The Sands of Time in the Prince of Persia trilogy work like this. Once released, they instantly turn everything they come in contact with into a crazed Enemy to All Living Things (living in this case being the few lucky souls who weren't instantly transformed). Somehow, when the Prince is exposed in the third game of the series, he manages to resist instant transformation, although he does still get a Superpowered Evil Side. The Corrupted, from the sequel, are an aversion-it's pretty much their own damnn fault they're Eldritch Abominations now.
The R-Type series eventually evolved the Bydo into something like this, although this isn't their favored modus operandi: usually, they simply evolve, replicate, and reproduce to improve themselves and strengthen their numbers, but given the opportunity, they will assume forms that are designed to infect and assimilate enemy technology and personnel. R-Type Delta had a good example of this in several of its ships and one of its Multiple Endings, and R-Type Final and R-Type Tactics/Command use this as a turning point in their respective storylines (much more the latter, with it even affecting gameplay).
In Over Blood it's called the ARNA Virus and Raz has it.
Resident Evil is replete with these, both in the form of literal viruses, and the Las Plagas parasites.
Knights of the Old Republic has rakghouls, deformed mutants with infectious bites, living in the Undercity of Taris. The player character cannot be infected, but several infected NPCs are seen transforming. In some cases it can be prevented with a dose of rakghoul serum, but once the victim has actually been transformed they must be killed. Interestingly, when the player character or a party member is bitten, they can be poisoned and take standard poison damage, and there is an instance where you save an NPC and he has been bitten and poisoned, but no one comments on this. This may mean that the rakghoul disease can be avoided outright if treated immediately. In the comics, it was revealed that the rakghoul disease was concocted by a Sith who made a talisman that instantly transformed humans into rakghouls which he could control, getting them to use their old skills, like weaponry. Comics taking place between the movies of the original trilogy had a fallen Jedi using this talisman on various heroes, including a member of The Remnant.
Extermination revolved around this. The Virus would mutate and corrupt pretty much anything to do with water. The player probably would freak out about the time a puddle of water actually attacked/infected him. Getting 100% Infection and not curing it fast leads to a Non-Standard Game Over where the player watches the transformation into a monster...
The Beast subversion entity from the sequel certainly counts. It quite literally rips its host apart, extracts their neurons, and then sets about using them to form an organic computer network it can subsequently use to control the ship they were flying, and hence infect more vessels. The quote formerly at the top of the page comes from the initial cut-sequence describing the entity — spoken by a poor engineer who sounds like he either wants to throw up or cry just about all the way through.
Ordinary Beast entities have an animalistic behavior and only care about multiplying. The original entity inside the Naggarok however, is very much sentient: when the Imperialist Taiidan make first contact with it, the entity offers them half the galaxy if they repair the Naggarok's drives. It also tricks them into believing that the Bentusi are gone and that they will get the Nomad Moon. When the Moon is infected and is subsequently destroyed by the Somtaaw who in turn receive aid from a Bentusi ship, the Imperials flip out and turn against the Beast; cue the Naggarok calling in all of its "children" to catch the ''Kuun-Lan'' from three sides.
Silpheed has an entire ship — the ship that refueled you about ten seconds ago, to boot — get taken over by an alien virus and turns into not only an enemy alien, but also That One Boss.
The old Area 51 lightgun game revolves around an alien virus being unleashed and causing a Zombie Apocalypse in the area. The Game Over screen shows a video of the protagonist morphing into one of the alien mutants.
Dead Space 's Necromorphs are unique in that only one of its forms is The Virus and it can only infect dead people. It's the task of the other Necromorphs to ensure there are dead bodies to infect. The Virus can somewhat infected people but only after weeks of being in contact with the source or at least being around the marker, drive them insane and kill themselves, after a few more hours the virus takes effect. The infector verson just speeds up the progess form 10 hours to 10 seconds.
One of the glitch Pokémon is Charizard 'M. Charizard 'M can also change all of a player's party Pokémon into Charizard 'M, but the moves and type do not change, and putting these "transformed" Pokémon into a box makes the other Pokémon Charizard 'M also.
For that matter, the Pokémon series has an actual gameplay mechanic called the "Pokérus", which your Pokémon can pick up randomly during the game. This isn't a bad thing by any means though, and actually helps your Pokémon get stronger stat boosts when they level up. It also wears off after a few days. They stop spreading it after a few days, that is. Any Pokémon that's ever had the virus will retain the speedy growth forever. It also "freezes" when the Pokémon's in a box, allowing you to keep a few virus-spreaders handy for when you need to infect more of your mons.
Yamask and Cofagrigus's ability, Mummy, is this. When it is touched by the opponent their ability turns to Mummy. Potentially, this can cause a lot of Pokémon to be "infected". However, this is temporary, and wears off when the Pokémon returns to its PokéBall.
Star Fox Assault has the Aparoids, a robotic insect colony from another dimension that infects both biological creatures and mechanical objects. A prime example of a victim is Pigma Dengar, who is fully assimilated by the aparoids, effectively killing him before he is destroyed by Fox. General Pepper nearly suffers the same fate.
Alma does this to Sergeant Keegan in F.E.A.R.: Project Origin, using her psychic powers to literally make him fall in love with her. The only way to save him is to kill him.
For a technological example: the unnamed computer super-virus from Descent, which takes over mining robots and turns them against humans, while at the same time drastically increasing their AI. It's much more powerful in the novel adaptations, in which the virus can take over just about any machine and give it sentience, and possesses AI of its own, allowing it to intelligently react to and avoid anti-virus measures.
The strange virus from Puzzlebox boosts a person's resilience to injury, inspires creativity and drives its hosts mad.
The Zerg from Starcraft, obsessed with assimilation into the swarm, pretty much embody this trope. More obviously, they can also infest Terrans (who either are Suicide Bombers or become Queen Bitch of the Universe).
In the StarCraft II mission "Outbreak", it morphs them into straight-up zombies, complete with moaning "Please... Kill Me..." the multiplayer/skirmish Infested Terrans lampshade it, complaining of "being under the weather" or "having caught something".
Emperor: Battle for Dune. The Contaminators, mutants spawned from Tleilaxu Flesh Vats. They carry a lethal virus capable of turning humans into additional Contaminators. There's also the non-human variant called the Tleilaxu Leeches, biogenetic tanks that create replicas of themselves by implanting larva in enemy vehicles, which damages the host vehicle until it's destroyed before hatching into another Leech.
The Bydo from R-type is probably one of the most horrifying version of this when you think about it hard enough. Due to their nature, they can turn themselves into a wave, meaning they can bypass any sort of protection without notice before they start taking over and can also take over machines with ease as well. This is what happens to you in one ending in Final. The thing is that you don't even realize you were infected by it.
The Darkspawn taint in the Dragon Age series. The Darkspawn spread this disease wherever they go, turning any living things that are exposed to it into horrible monsters. Yes, this includes people. The taint unites the Darkspawn in a primitive Hive Mind, though only an Archdemon, a draconic Old God corrupted by the Darkspawn taint, can unite all of the Darkspawn hordes. That's right, this particular Virus can corrupt gods.
Both System Shock and its sequel feature enemies infected by The Virus. In the first game the hybrids are subservient to SHODAN, but in System Shock 2, they're controlled by the Hive Mind of "The Many".
In Strife, the virus arrived via a massive comet impact. Those who weren't wiped out begin to mutate, hearing the voice of an alien monster and causing their bodies to rot. The Evil EmpireReligion of Evil that worships the monster uses cybernetics to maintain their self-destructing bodies.
Overlord has an example of the "turns the victim into a mindless shambling zombie" type. The interesting part is how it spreads: tracing it to the source in Haven's Peak will reveal patient zero to be a Succubus Queen. The plague's a staggeringly virulent STD.
The virus blocks in Astro Pop. They can't be grabbed, and you can only get rid of them by making a match next to them. They also "infect" other blocks, turning them into more virus blocks. You can interrupt the transformation if you grab the block before it's transformed.
The plot of Abuse revolves around a mutagen of the same name created from the gene that causes violence and aggression in humans, which mutates them into grotesque Xenomorph-like creatures.
The zombiesurvivalsandboxThe Dead Linger takes place after a virus called HNZ turned the majority of people into zombies suffering from necrosis, reduced intelligence and a hunger for human flesh. The only good thing is that they become slower and eventually die off if they "live" long enough. But the really bad thing: The player is not immune!
Kuru is a very real disease that stems from cannibalism, and can invoke a zombie like state. Kuru mutating to Hell on Earth proportions is the basis for Dead Island.
A meta example: Some games with multiplayer modes offer a achievement that is "earned" by playing online with or against another player who already has it, starting from a very small initial group of "infected". E.g. "Six Degrees of Schafer" in Brutal Legend started off with just Tim Schafer having it, while "Bubonic" from XCOM: Enemy Unknown gets Bonus Points for actually using the viral theme in its name.
The revenants in Girl Genius. A bioweapon devised by a mad scientist, the revenants are produced by devices called hive engines. When activated, the devices release a queen "Slaver Wasp" that almost immediately produces hundreds of soldier wasps (insectoid, non-flying killing machines the size of a big dog). They then make a perimeter and drive everyone inside to the center, where the queen releases a horde of tiny wasps that fly down your throat and control your brain. These newly created revenants act as zombie guards for the hive while the warriors corral in more humans. Additionally, the revenants are completely loyal to the creator of the hive engine, making them an excellent device for assembling instant armies of mooks. The revenants are the exception. Most people infected go about their lives normally... but completely obedient to the Slaver Wasp creator.
The Dimension of Pain demons from Sluggy Freelance possess a spear that can turn human beings (and ferrets, apparently) into demons, and they use it frequently during their invasion of the Dimension of Lame. However, since the spear can't completely get rid of the humans' inherent wussiness, the result is an army of pretty wussy demons. A human-made version (or is it...?) has appeared in the main dimension now. None of the resulting demons has been seen for more than a strip or two, so it remains to be seen if the results are Lame or otherwise weaker than real demons.
The Legion from MSF High are a legion of Green Skinned Space Babes who, when forced to fight, turn their attackers (whether male or female to begin with) into Green Skinned Space Babes who see things their way. That's in the present; they worked similarly in the past, but replace "attackers" with "defenders" and "see things their way" with "lose all of their original personalities and fight mindlessly for the Queen."
The Dragon Doctors face their first real challenge when attempting to cure a patient who has a "Crax Parasite" inside him, a horrible, cancerous growth that will eventually consume his body and mind. The four magical doctors each make use of their full abilities to cure the disease. Kili the shaman occupies the patient's mind in a dream world while the others operate, and it is revealed when the Crax invades the dream that it's the vehicle of the consciousness of a man who wants to live forever.
In Superidol computer-generated idol Rei Rei is the memetic version of this, with billions doing everything they can to be her, until the whole world revolves around her.
Zombie Waffe has LARS (Likasi Acute Rabies Syndrome) a new form of rabies which turns those infected into zombies.
Unity, one of the protagonists of Skin Horse, is a non-virulent virus. She's a collection of nanobots suspended in fluid, and anything injected with her becomes her completely... but she can't actually fight off the immune systems of the living at all, so anything not dead she tries on ends up vomiting her out after a short while. And she has no method of spreading outside of purposeful injection.
The Kingfisher is a vampire comic in which the condition can be communicated. It usually doesn't affect the mind of its victims directly, unless they become too hungry for blood.
The Global Guardians PBEM Universe included the aptly named Virus, a villain who takes control of people using a tailored genetic infection that slowly transforms them into copies of itself. Its ultimate goal is to make all life on earth a copy of itself.
The Infestation of The Nutty Joes. A giant brain crash lands in the middle of a quiet country village, hatching a crazed creature whose laughter soon rings out throughout all of mankind.
Blights are deranged intelligences which try to consume and convert all minds and processing platforms they can reach into likenesses of their own mind. Many of these occur when the original mind attempted transcendence, which has the unpleasant side effect that the Blight is also a transapenient with all of the godlike mental abilities that implies... being able to totally outthink and control its former peers being perhaps the least of their new skills. The Amalgamation is a horrifyingly powerful blight capable of subverting even powerful Archai.. hugely capable god-like intelligences that are vastly better equipped to defend against Blights than mere mortals.
Affines are a sort of sentient virus for highly advanced transapients. Hive Mind type transapients with subminds dealing with perception are most at risk... imagine your eyes and ears became malignly sentient because of something you saw or heard and then went on to slowly trick you into doing what the virus wanted. Human sensory systems are entirely too primitive for affines to work on them, but the seemingly benevolent AI overlord of a planetary colony might one day start turning into a brutal dictator and slaughter huge numbers of its former citizens without a care.
Beautiful joke example from The Tick: when reminiscing about past adventures, Captain Decency remembers the time a villain built a superweapon known as the "Ray Gun". This gun functioned by turning anyone it shot into a duplicate of "some guy named Ray", depicted as a gas station attendant with the nickname "Ray" on his coveralls. Cue incredibly creepy Twilight Zone-style black-and-white shot of a town filled with identical Rays saying "Hi, I'm Ray!" to each other over and over. Captain Decency then begins to reminisce about a similar adventure involving a "Tommy Gun" before being cut off.
In The Pirates of Dark Water, the eponymous Dark Water had the ability to kill people outright, or corrupt them into mutated minions or mindless slaves. In one of the more Nightmare Fuel-laden episodes, an elderly woman tries ingesting a drop of it in a youth potion and it winds up consuming her completely from the inside.
Daemon was a literal software virus in ReBoot as were Megabyte and Hexadecimal. Daemon and Megabyte match this trope better as they had the ability to infect the system's sprites and convert them into their loyal soldiers.
In Code Lyoko, some of XANA's attacks follows this route, the specter first possessing one animal then spreading the control to others (rats in "Plagued", wasps in "Swarming Attack"). In "Attack of the Zombies", a possessed Kiwi can transmit The Virus to humans, Zombie Apocalypse-style.
A robot from a planet of robots fears humans are The Virus in the Futurama episode, "Fear of a Bot Planet".
Robot: Is it true, humans sneak into your room at night, drain your fluids, and turn you into a human?
The Brain Slug:
Hermes: The flight had a stopover on the Brain Slug Planet. Hermes liked it so much he decided to stay of his own free will.
Played for laughs in an episode of Camp Lazlo. It's learned in this episode that when Samson gets sick, his germs can cause others to at first, get sick, and then end up looking a lot like a hamster who is sick.
The Xenocites transforms carbon-based lifeforms into a DNAlien under the thrall of the Highbreed. Thankfully, the Omnitrix has the power to revert the genetic damage and restore the original form of the victims.
The nanitehive from the Alien Swarm movie is another example.
The original Transformers has the Hate Plague from Outer Space which was unleashed upon the Earth. At first it appears to make those infected fight others and themselves, but later on it just appears to make them act like jerks with the intent of infecting others. Either way, the last known hope to the world, nay, universe is to revive Optimus Prime. It works, of course, and even Galvatron is thankful enough to call for a truce. For now. Previously in the same season, the Transorganic energy-leech threatened to spread the robotic equivalent of vampirism to all of Cybertron. Much later, in Beast Machines, technomatter made with the Key to Vector Sigma had much the same effect - when the Maximals were zapped with the Key, they became both contagious and mentally unstable, and tried to infect each other.
The Fudd from Duck Dodgers, a parody of the Flood which transformed people into goofy-looking bald men with speech impediments.
In the Regular Show episode "Guy's Night", anyone who drinks a gallon of milk without holding the empty jug over the head is sent to a white room full of white creatures that turn anyone they bite into one of them.
Viruses, of course. They tend to do it on a microscopic scale; a popular method is for a virus to inject its genetic material into a cell, which corrupts/mutates it, making it produce more viruses inside itself. It then bursts, spreading many more viruses throughout the body and killing the cell in the process.
Bacteriophages work the same way - being viruses that target bacteria.
Ebola does the same thing. Only to humans, at the human level. Richard Preston's The Hot Zone provides very graphic descriptions of what it does to its host. The scariest part is that it's non-fiction.
Rabies. It infects your brain, drives you mad, uses you to spread the disease, and kills you once it's done with you. Yup, a real-world equivalent of the Rage. Fortunately it tends to kill within a week of showing symptoms, so your chances to spread it are limited.
Kuru: a real disease that leads to zombification and eventual death.
Prions are proteins that act as The Virus to other proteins, converting them into prions. This can also become The Virus for organisms, if the proteins being converted happen to be located in the brain.
Diseases can alter your behavior in ways to spread themselves, but you may not realize it. Malaria makes you too tired to fight off mosquitoes, for instance. Defensive reflexes designed to rid our bodies' airways of infection (coughing, sneezing) are also behaviors that tend to spread it to others.
Some real-life Puppeteer Parasites do this. Some require one host to carry, and another host to reproduce in, and force their primary host into zombie-like behavior. Some even have ants climb onto grass stalks overhanging their hives and emit spores as they decompose to infect healthy ants. Sounds like Resident Evil 4meetsA Bugs Life, doesn't it?
If certain theoretical conditions are met, stable strangelets — a hypothetical category of subatomic-scale particle — could convert any normal matter they contact into more strangelets, which could then go on to convert more normal matter, and so on.
Botnets are a type of Computer Virus that are designed to do this-every computer a botnet infects is assimilated into the owner's network and becomes a tool for sending out more infected emails and whatever they require the processing power for. Bonus for the actual computer that directs the botnet being called a master control. A slightly older terminology calls the infected machines Zombies and either the controlling computer or the human(s) controlling it the Zombie Master.
The word "infection" itself is something of a virus. Pay attention to the terminology used when holding conversations with a person about a story involving some kind of monster assimilating or impregnating it's victims; Chances are they will use the word "infection" instead of the more accurate words. Indeed, "infection" is spreading and killing every other similar word in the English language, and there's nothing you can do about it! Woe is the grammar Nazi! This is probably due to the vast popularity of zombie stories in the last decade or so.
The Cordyceps genus of fungi grows in insects. It eventually kills the host by fililng up its body cavity. Often, at the very end of the process, it forces its dying and deranged host to crawl to the top of a nearby high point and cling there until its death, at which point the fruiting bodies of the fungus emerge and spread their spores.
Sacculina is a parasite of crabs. The larval female form burrows into a crab, sterilises it, disables its regenerative mechanisms (because these use up nutrients that Sacculina wants for itself) and pokes a bit of itself out of the crab's thorax waiting for a male Sacculina. When the parasite's egg have been fertilised, the parasitised crab looks after the Sacculina eggs as if they were its own.
A very ironic case is swine influenza, or "swine flu". Many people think that they can catch this dangerous disease from infected pigs, and fear of it has led some countries to mass slaughter them. In truth, humans cannot catch this disease via casual contact with pigs (humans who work in that industry who butcher them might), but ironically, pigs can catch it via casual contact from humans.