"Of all the parasites I've had over the years, these worms are among the — hell, they are the best!"In biology, a symbiote (or symbiont) is a living organism that lives in symbiosis with another organism. Symbiosis literally means "living together," and it comes in three variations:
— Fry, Futurama
- Mutualism - both organisms benefit from each other's presence.
- Commensalism - one organism flourishes, the other isn't affected.
- Parasitism - one organism is harmed, the other flourishes.
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Examples of Mutualism:
Anime and Manga
- In Kurau Phantom Memory, the Rynax-entity merged with Kurau's body benefits from the protection for her recuperating pair, while giving Kurau immense powers in return. The finale reveals that the original Kurau was just a bystander. After the Rynax leaves her body, Kurau tells her father she can remember everything that happened when the Rynax was controlling her, and when she talks to Christmas she says she's not the Kurau Christmas knew, and that even though she isn't the Kurau Christmas knew, she saw everything the "other" Kurau and her went through together, which means the original Kurau was just an observer, watching the Rynax's life in her body until it left. Though, she did seem to have fond memories of the experience.
- In Pokémon, a Shellder latches onto a Slowpoke permanently to create a Slowbro or Slowking.note The Slowpoke gets stronger by way of evolving, and the Shellder gets to snack on the Slowpoke's dinner scraps. Or something.
- In Macross Frontier, the Vajra have a special kind of bacteria in their intestines that maintains the telepathic connection between them. If the bacteria gets in humans (rare, as it requires ingesting of bodily fluids), it slowly kills them as a disease known as a "V-Type Infection". It's eventually revealed that Ranka Lee has these same bacteria in her intestines, and due to contracting them in utero she is fully symbiotic with them. They allow her to tap into the Vajra Hive Mind, making her the only human being who can communicate with the Vajra. At the end of the series, she uses her connection to cure Sheryl's V-Type Infection by influencing her bacteria into a similar symbiotic relationship.
- Kill la Kill has the Kamui, sentient clothing that feed on their wearer's blood, and in return give them superhuman strength and resilience. May overlap with parasitism, as Kamui can cause exhaustion and Body Horror. Episode 16 reveals that this applies to all life fibers; they're aliens that feed on humans and have helped them evolve so they would eventually wear clothing made of life fibers.
- The color entities of Green Lantern feed on emotion and allow their hosts to manipulate solid light on a much larger scale than power rings. Unfortunately, some of them affect the behavior of their hosts, which can vary from subtle influence to full on Puppeteer Parasite.
- In The Incredible Hulk's "Crossroads" arc the Hulk went to a planet where the food is poisonous to animals unless they have a symbiote attached.
- In Generation X, the mutant known as Japheth, sort of. His mutation turned his stomach into a pair of symbiotes that could leave his torso to eat for him, then return to provide the nutrition to him, along with a temporary super-metabolism (super-strength, slightly enhanced healing, etc).
- The Symbiotes from Spider-Man, although they have gained a tendency to turn their host evil since their first appearance. The ones in the Ultimate universe are parasitic, though.
- The core universe Symbiote species is as well (at least, after 90s retcons). The Venom Symbiote and its spawn are considered mutants by the others. Or they were, before they completely wiped out their Always Chaotic Evil brethren.
- The Venom symbiote is coming back to this, after bonding with Flash Thompson, realizing that they must rely on each other to survive.
- The Toxin symbiote, when bonded to Patrick Mulligan, had a relationship similar to that of a father and a naughty little child. Not so much when Toxin is bonded to Eddie Brock.
- The Carnage symbiote, when bonded to its first host Cletus Kasady. The symbiote and Cletus genuinely seem to care for each other and feel incomplete unless they are together. It's a body controlling parasite to anyone else, whom it only uses to help it reunite with Cletus whenever they are separated. Cletus tells someone who tried to steal the symbiote from him that he and Carnage were pretty much made for each other.
- The Venom symbiote truly wanted to be this for Peter Parker, its first human host. It wanted to be with Peter and it wanted to make him stronger. While the symbiote had the same corrupting influence on Peter as it does on other hosts, it does so not out of a desire to control him but because it genuinely believes it's doing Peter a favor by getting rid of his compassion.
- The symbiotes from Spider-Man may have inspired the eventual revelation that Spawn's distinctive costume (which is standard issue for all Hellspawn) is actually a living, sentient demon in its own right. How the "costume-demon" benefits isn't made clear, possibly from being able to passively feed off of the necroplasmic energy of its undead host. The host definitely benefits from having a very protective live-in partner which not only provides flight/gliding (the cape functions like wings because, in its natural form, they are wings), offensive shapeshifting and Combat Tentacles (via the chains and cape-tendrils), but will fight to protect its host even if the host is unconscious. Said symbiote feeds of negative emotions and pain, being around Spawn of all people is giving it plenty of that. In an early issue it's described as partially feeding off the necroplasm his body is made of, and in a later arc where Spawn is convinced his costume is responsible for a grisly seies of killings where the victims were drained of all their blood, clown reveals it feeds off souls (of which necroplasm is a condensed form). To no one's surprise, the murders were being committed by a vampire.
- In the Pony POV Series, Changelings that form genuine love with ponies generally end up like this. Changelings need love to survive, and will quickly die if they don't get it. So when a Changeling gains a genuine bond with a pony, the pony gets a loyal loved one out of the deal (something ponies appreciate) and the Changeling gains an infinite source of love (as opposed to when they take love, which generally sucks the target dry and thus is finite). In general, Changelings who do that is much better off than those who act as parasites. This is taken Up to Eleven in Dark World, where the Changelings performed a Heel–Race Turn and form a large part of La Résistance against Discord, since Cadence's magic still infusing them allows them to No-Sell Discord's magic.
- Star Wars:
Qui-Gon Jinn: Without the midi-chlorians, life could not exist, and we would have no knowledge of the Force. They continually speak to us, telling us the will of the Force. When you learn to quiet your mind, you'll hear them speaking to you.
- The Tusken Raiders of Tattoine ride giant, shaggy beasts called banthas, which share a near-supernatural bond with their owners via a combination of training and affinity. A bantha is always the same gender as its rider, and when two Raiders marry, their bantha's also become mates. If a bantha's rider dies, the bantha usually dies shortly after, often becoming feral and violent before dying.
- The midi-chlorians:
- And You Thought Your Parents Were Weird! has Matt, a ghost, possessing Newman, a robot. Matt needs Newman's body to interact with the world, and Newman benefits from having a smarter mental roommate.
- Growth has leech-like parasites that infect a human host with their larvae and make their hosts stronger and smarter. However, the larvae rapidly mature and take over the body, reproducing and making their hosts aggressive. They also cause salt to act like acid. Often, mature parasites will Zerg Rush humans when hungry. Rather than infecting and reproducing, they eat.
- Men in Black 3: Boris the Animal has a symbiotic relationship with a small creature that burrows into his right hand so he can shoot lethal spikes at a very fast rate. Smuggling his symbiote into prison and giving him free use of his hand is what initially allows him to escape his confinement.
- The Iskoort in Animorphs.
- The Yeerks too, who are like the Goa'uld in that they skirt the line between mutualism and parasitism. They forcibly take hosts, but it's just as beneficial for them, as in their normal states they're blind, powerless slugs. The Iskoot are what happen when the relationship goes over to full mutualism, hence why the Crayak wants them destroyed before the other Yeerks find out and spoil its fun.
- The Yeerk resistance movement is kinda like the Tok'ra in that host and symbiote share the body equally.
- Similarly, the symbiote in David Weber's book The Apocalypse Troll kills over 99% of its hosts, though in this case it's justified since it was originally developed as a bioweapon, and intended to kill 100% of its hosts. Those who survive, though, gain effective immortality via Healing Factor (and eternal youth into the bargain), enhanced senses and reaction times, and generally superhuman abilities.
- The symbiote from Anne McCaffrey's Crystal Singer novels has a low success rate for adaptation to human hosts. Those who survive, though, gain a Healing Factor that makes them virtually immortal, barring murder or immediately-lethal accident. Too bad about the slow memory loss, dementia, and paranoia...
- In John Varley's Eight Worlds series, there are the symbiotes; artificially cultured plant-based organisms that are bonded with humans to produce a single organism that has its own individual animal/plant ecology. They don't breathe or eat, and spend their time in open space, usually touring the rings of Saturn.
- Likewise, the cave slug in F. Paul Wilson's Healer is believed to be 100% fatal, but the title character is one in a thousand and instead gains the "usual" benefits per the two examples above, along with a voice in his head (which he names "Pard", as in partner). His touch can also heal others of physical and mental illness, making him a figure of awe and legend.
- The Binod Union in Atavar, so much so that they're considered a single race.
- The Hitchhiker's Guide to the Galaxy's Babel Fish, a living plot device as a Symbiote.
- The vampire-fungi from the Necroscope series technically fit here. The fungal-leech gets a host with sentience and opposable thumbs, the host gets enhanced psychic powers including shapeshifting), enhanced strength, enhanced senses, and a craving for fresh blood and human flesh...
- The Kualkua species in Sergey Lukyanenko's The Stars Are Cold Toys duology are shapeless creatures able to split by mitosis. They are used by the Conclave for anything from translating speech in real-time to piloting suicide ships. At first, the human protagonist is horrified (but not surprised) as to the treatment endured by the Kualkua at the hands of the Conclave Strong races. Later, he is horrified after learning the true nature of the Kualkua (that of a Hive Mind, which to him represents near-godhood). This also crosses over into Commensalism and Reverse Parasitism.
- The War Against the Chtorr. A characteristic of the invading Chtorran ecology. For instance Chtorran gastropedes are covered in neural symbiotes (so-called 'worm fur') that vastly increase their senses, making them super-efficient predators. Shambler trees are host to over thirty different species of carnivore that can seek out and devour prey, passing on nutrients to the shambler via their waste products.
- The K'da from Dragonback. Basically a tiger-sized dragon that can turn into a tattoo, and must do so at least once every six hours. The K'da gets a host, the host gets a powerful guardian, and both get the other's companionship.
- In Octavia Butler's Fledgling, vampires (or Ina) are like this with humans. In fact, the humans they feed on are called their symbiotes. Ina feed on human blood, and humans are addicted to whatever chemical is in the Ina's saliva. The humans also get to live longer (but they can't be turned). There even is a Hemo Erotic part, with Ina often having sexual relations with their humans.
- This shows up at least twice in Poul Anderson's Technic History series: one encountered briefly by Nicholas van Rijn, and the natives of Dido in The Rebel Worlds, who are a combination of three species—the "hands," the "wings," and the "feet."
- The detective-creature in Hal Clement's books Needle and Through The Eye Of The Needle was a blob of protoplasm that entered a human host to survive and move around. It was a type 2 (commensalistic) in the first book as the host was not harmed, but shifted towards a type 3 (parasitic) when the host became ill in the second book. There was also another creature, the hunted fugitive, who'd taken another body and was a Puppeteer Parasite type. Clement actually coined the word "symbiote" in Needle. He later apologized for this, after biologists pointed out to him that the correct word is "symbiont"; it was too late by then, as a number of other writers had copied his term.
- Galaxy of Fear: Eaten Alive: Enzeen on D'vouran, after The Reveal, are repeatedly called parasites. The planet is alive and eats people, and they feed from it. However, it's clearly mutualistic; the Enzeen make visitors feel welcome on D'vouran and help hush things up whenever people start to feel suspicious and might start figuring out what's happening.
- The Rhumians in the Sector General novel Code Blue Emergency are almost inert brain-creatures who form a symbiotic relationship with a non-sentient species from their planet, but can link to almost any living thing in emergencies. The squickier elements of this are acknowledged in canon, with even the radically-accepting medics of Sector General initially mistaking them for evil Puppeteer Parasites.
- The Stormlight Archive: The Nahel bond between human and spren (essentially a sentient idea) is a slightly odd example of this, since spren are incorporeal. The bond grants the human Surgebinding powers, and allows the spren to retain its sentience in the Physical Realm (spren are native to the Cognitive Realm). If the bond is broken, the human loses access to Surgebinding, while the spren becomes mindless.
- The nameless worm-thing living inside Roger Harding in The Behemoth is an example of this, though it edges toward parasitism — it gives Roger super-powers, including the ability to transform into the titular Behemoth, but grows whenever he uses his powers. It is left vague how large it might grow and how completely it might take over.
- Mobium in Superheroes Anonymous is a semi-sentient Super Serum that provides powers to its recipients while rebuilding their body to be more powerful. Part of the rebuilding is replacing much of the organic tissue with more Mobium, which allows it to spread itself.
- The alien needle symbiote in Eden Green is mutualist: The needles are able to spread to new life forms, and the hosts gain nigh-immortality.
- Treecats in the Honor Harrington novels look something like six-legged house cats. They are essentially Bond Creatures, and removing one from their human results in both pining away.
- In the Star Darlings series, Starlanders grant Wishworlder's wishes to keep their planet alive. Without positive wish energy Starland would die out, and without granted wishes Earth would be a depressing place to live.
Live Action TV
- The Vindrizi from "Exogenesis," in season 3 of Babylon 5. A race of living recorders designed to preserve the memories and knowledge of their creators, the Vindrizi seek out voluntary hosts who have nothing of their own left to live for.
- In Fraggle Rock the Fraggles have an odd symbiotic relationship with the Doozers. Doozers build large structures and other constructs with a candy-like substance, which is soon eaten by the Fraggles. An episode shows that if the Fraggles don't eat the Doozer buildings they eventually grow out of control and are left with nothing else to build, and the Doozers actually like the Fraggles eating their buildings as it lets them know their hard work is appreciated.
- Stargate SG-1: The Goa'uld and Tok'ra are tiny, snakelike aliens who can control human hosts by taking residence somewhere in their neck. The host benefits from an extended lifespan, Healing Factor, and boosted strength. The difference between the two factions is that the Goa'uld take hosts against their will and don't intend to relinguish control, making it explicitly parasitic. The Tok'ra, except in cases of emergency (like a symbiote being near death), will ask permission first and share control with their hosts.
- The Trill in Star Trek: Deep Space Nine. The humanoid Trill host keeps the vermiform Trill symbiont alive, while the host enjoys the cumulative memories (including skills) of all the symbiont's previous hosts.
- Very similar to the Vindrizi, the Kergillians from the Over the Edge tabletop RPG (and related card game).
- One example in Rocket Age is the Fur Frog, a small creature that Venusians keep as pets. They do eat the parasites living in their host's fur, but more importantly they are highly psychic and effectively grant their host access to these same abilities.
- Peacebringers and Warshades in City of Heroes. They're each a kind of Kheldian, Energy Beings from outside our galaxy who can merge with humans; the humans get powers, and the Kheldians get immortality as long as they have a host; their "natural" lifespan is only ten years. They're an unlockable playable class (well, two Archetypes to be precise), and their powers include Shapeshifting into their previous common forms, including a floating tentacled Glass Cannon and a large armoured Stone Wall, nicknamed the 'squid' and 'lobster' respectively.
- In Knights of the Old Republic II: The Sith Lords, a psychic connection of this sort is formed between the player character and Kreia.
- The fierce Harika and mouse-like Yorn in Star Control 3.
- Skullgirls has loads of examples of numerous different types of symbiotes, though they're all referred to as "parasites" in-game. Of the playable cast:
- Filia has a symbiote living in her hair named Samson, and he zigzags between mutualism and parasitism: while they do work together in combat, Samson has a tendency to take control of Filia in some of their moves, and it's hinted that he wants to turn Filia into the next Skullgirl—a Person of Mass Destruction who ends up being the main agent of the Bigger Bad.
- Squigly, meant to be The Rival to Filia, has a symbiote who act as a rival to Samson: Leviathan, a snake-like creature who has a much friendlier relationship with his host than Samson does.
- Eliza is the host of a symbiote named Sekhmet, who is different from the previous examples in that she has entirely replaced Eliza's skeleton in exchange of granting Eliza eternal youth as long as she feeds on blood.
- The compacts between spirits and humans in Poacher are this.
- The Gnoph in Gnoph live inside human lungs, and grant their hosts a variety of superhuman abilities.
- The Accidental Space Spy: One of alien spies have this, the real body is just a oval. Rest of parts such as hands, eyes, etc were parasites but later adopted to mutual status.
- Work Sucks: Epicena has a symbiotic monster named Hatch hooked to his...her...its back that has the ability to inhale and capture people Epicena wants. Epicena says Hatch is harmless unless (s)he tells it otherwise.
- The Worms in the "Parasites Lost" episode of Futurama (see the page quote.)
- Symbiotes in the Men in Black animated series. Basically, they were metamorphic heads which had to attach to other creatures to survive. Having one attached to you meant you got super strength, shape shifting, and highly powerful regeneration powers; unfortunately, the only one we see is ungodly annoying and clingy, and if you're bonded with one past a certain period of time, it becomes permanent.
- The series has Headmasters, Targetmasters, and Powermasters. The human partners are kind of like having a gunner in the first two cases, while the human partner gains the armor and weapons of a Transformer. With Powermasters, the human partner serves as the engine on a planet when all fuel sources were poisoned to keep the Transformers' battle away from them. The bad news is that both give up some autonomy.
- There are also cases where a Transformer forms part of a larger one. Being carried around by (and presumably getting to lap up some spare Energon from) a big guy like Trypticon, Metroplex, or anybody with Maximus in the name is really handy, and the larger TF gets a small army of normal-sized Transformers that are always at his side, and who can handle fine-tune work. On the rare occasions on which Trypticon actually doesn't want to break stuff, he can sit back and send in his little buddies. These guys' status as fully sentient characters vs. remote-controlled tools changes between incarnations.
- In PJ Sparkles, PJ is a mutual version of the trope, as she gives other kids the love they need but needs them to love her in return to survive.
- You've got some in your intestines right now, people. They're gut bacteria that eat leftover fiber from your food, and pay their rent with the vitamins they excrete. Lose them due to radiation therapy, high-dose antibiotics, or other causes, and your doctor will make you eat live-culture yoghurt until their population levels are restored.
- A very particular case of real life symbiotes are mitochondria, which actually live inside the cells of practically all animals (including humans), playing an important role in the metabolism.
- Plants have something not dissimilar in their chloroplasts, the organelles that photosynthesize. These too are theorized to have been symbiotes at one point, and which are now completely interdependent.
- It's also theorized that eukaryotic cells in general (cells with nuclei and other complex internal structures) are also the product of a symbiotic relationship.
- Those white lumps seen in pictures of humpback whales are actually this trope, as they're giant barnacles that grow nowhere else but on the hides of baleen whales. The otherwise-immobile barnacles get a free ride into plankton-rich waters, at which point they start waving their net-like appendages to feed. The whale can hear which flipper's barnacles are scooping up plankton more rapidly, and turn towards the direction where food (for whale and passengers both) is most abundant.
- Many "cleaner" species, such as tickbirds or cleaner wrasses, pick parasites (see below) from the surface of larger animals for a living. They get a meal, and the bigger animal gets a de-lousing. For instance, Oxpeckers walk a fine line between mutualism and parasitism, since their main diet is their hosts' blood—they get some of it from the fleas and ticks they eat off of their host, but they will also sometimes peck at their hosts, or reopen their wounds, to get them to bleed.
- One theory on the origin of complex life is that a bunch of single-celled organisms joined together and skin and bones/exoskeleton were a handy defense that developed over time.
Examples of Commensalism:
Anime And Manga
- The Guyver units from Guyver.
- In Bazooka Jules the source of Jules' superpowers is a micro-robotic weapon called the symbiote that entered into Julie's body and permanently fused with her nervous system. It has two main functions. One is to enhance its host physical abilities with chemicals and hormones making them stronger, faster, and more durable. The other is to provide its host with weapons and gadgets. It was has various detection systems, a radar, and can provide its user with tactical advice, hence the voices inside Julie's head.
- In Warhammer: Age of Sigmar, Curselings are gifted with a parasitic, daemonic homunculus known as a Tretchlet. These twisted spirit-creatures are created from an eldritch coalescence of forbidden knowledge that has gained sapience. These homunculi constantly whispers advice to its host and support them with their arcane abilities as the Curseling continue their search for further hidden knowledge.
- This form shows up in Knights of the Old Republic II as well: the Exile, having cut off his connection to the Force after all the death at Malachor, effectively re-establishes that connection by becoming a black hole in the Force, siphoning excess power from his companions. Taking advantage of this is what allows you to kill Darth Nihilus.
- The Cue Cappa Commonwealth in the Space Empires series consists of two species - the Cue are small, parasitic entities with potent psychic powers who latch onto the much larger Cappa, creatures with limited intelligence who perform manual tasks on behalf of the Cue. This relationship is thought to be millions of years old and influenced the evolution of both species. Most other species fear the Cue as dangerous parasites, but those who overcome this prejudice find them to be trustworthy allies.
- Remoras get transported along by their host animals, benefit from their protection, and possibly eat their leftovers. But they don't seem to do anything for or against their hosts.
- The Cyomatha exiguna kills and replaces a fish's tongue. The fish is no worse for wear, apart from now having a tongue with its own eyes and brain. It does use up a bit of blood, but usually not enough to matter.
- You probably have a lot of these on your body without knowing it, such as dust mites that eat shed skin cells or amoebas that feed on the bacteria at your gumline (the latter being a double example). A peculiar example are Eyelash Mites, which live exactly where the name would imply and emerge at night to consume miniscule amounts of the natural oil layer of your facial skin.
- Even a bird nesting in a tree can be considered this. Depending on their diet, however, they may be symbiotic by consuming the fruit of the tree and aiding reproduction by spreading its seed with their excrements.
- Cattle egrets often follow large mammals around and snatch up insects and small vertebrates that are scared out of their hiding places by said mammals' footsteps. The egrets will often perch on top of the mammals' backs to use as lookout posts and launch pads for better hunting. While this increases the rate of success for the egrets, the mammals don't seem to be helped or harmed by the process (the birds may occasionally eat parasites off their skin though).
Examples of Parasitism:
Anime And Manga
- The Parasitic Beasts in Parasyte.
- They drill into people's heads and take them over by eating their brains. Shinichi, the protagonist, manages to get away with one taking over his right hand (it aimed for his head, but he pulled it away in his sleep and it took over his hand in desperation).
- Mamoru Uda managed to be drowning when he became a host, making him also this.
- The symbiotes in Spider-Man are oftentimes portrayed as this. Incidentally, this was the original function of its species. Taking a host, driving them to an early death, and then finding another. The fact that the Venom Symbiote wanted to establish a life-time bond with one person made the rest of its race consider it to be psychotic.
- The Yeerks in Animorphs; who in their natural state are little more than breadloaf-sized gray slugs, who need our bodies to free themselves from their senseless prison. A branch of their species became the Iskoort (noted above) and it's stated that there is a chance they will follow that path as well in the distant future.
- In The Dresden Files short story Day Off, two of the Alphas are afflicted with "supernatural fleas".
- The Black Worm in John Connolly's The Cancer Cowboy Rides. An inhabitant of the Enemy to All Living Things, Buddy Carson, it grants its host the power to transmit fast-acting cancer at a touch- and enforces its use with agonising pain and the threat of infection. Though the Worm doesn't speak in the short story, Carson mentions a dream in which it tells him that his only purpose is to "Spread the Black Word."
- In Infected (the novel), the parasites are extra-terrestrial in origin and cause the growth of a new consciousness that encourages the host to kill and maim as much as possible. That's just a side-effect. Their real purpose is to guide humans to an area, where the parasites are really "workers" - they use the humans to build an organic teleportation gate for the invasion of Earth.
- Lotus in Monster is one of these feeding off the universe.
- In Peeps, vampires are due to a parasite living inside the body of a human host. The entire book is all about parasites, even with tips about real parasites at the beginning of each chapter.
- After Man: A Zoology of the Future had a species of shrew-like mammals that evolved into a parasite, using their mouth to suck blood from their host like a giant mosquito.
Live Action Television
- Another example of the parasites turn you into vampires route is a cestode parasite native to dodo birds in Primeval. Granted, they do provide you with a temporary boost in physical strength and endurance, but they also cause incredible levels of aggression, intense aversion to light, the compulsion to bite everyone you see, and ultimately death as the parasite matures from microscopic larva to foot-long adult in a matter of hours. There's no known cure other than surgical removal of the parasite, which all too often can't come quickly enough.
- Star Trek: The Next Generation:
- The original appearance of the Trill in the episode "The Host". They were given a makeover before Star Trek: Deep Space Nine. The relationship wasn't as bad as usual, but the character in question did refer to itself as a "parasite".
- Also, the host of the original Trill symbiont was apparently completely submerged - no sign of Riker in there when he was filling in as host. If the Trill hosts minded, it wasn't said, but the relationship between host and symbiote - symbiote is the person you're talking to, host is just a body it uses - is the same as with the villainous host-takers.
- Star Trek II: The Wrath of Khan had parasitic eels native to Ceti Alpha IV, which Khan used to control several Star Fleet operatives.
- The Black Oil a.k.a. Black Cancer a.k.a. Purity from The X-Files is an alien virus that gets into your body through your eyes and mouth and assumes complete control over it, optionally using it as a host for gestating a baby alien.
- Stargate SG-1
- The Goa'uld are a race of parasitic symbiotes. They are snake like beings who enter into a host and wrap themelves around the spinal column of the host taking complete control of their every action. The Tok'ra are a group of Goa'uld who despise this and enter into a symbiote/host relationship where both parties can take control.
- Anubis becomes this in season eight. He is a Goa'uld (see above) who figured out how to Ascend to a Higher Plane of Existence. The other Ascended didn't like that, so they kicked him halfway back down to the lower planes to punish Oma Desala for helping him ascend. He now exists as an Energy Being which needs either a force-field suit or a host to interact with the material world. In the latter case the host has no control over the body and reacts to him as if he were a disease, breaking out into lesions and gradually dying. (Mind, most Goa'uld qualify for parasitism, though they can choose to let the host out to play. Being half-ascended is what makes Anubis burn out his host; a lot of other Goa'uld hosts would consider that getting off easy.)
- The "Zombees" in Dead Rising. The cause of the zombie outbreak is a natural species of Colorado wasp that injects a host with larvae that travels into the brain and takes it over, turning the host into a zombie in which the larvae develops.
- The Necromorphs in Dead Space.
- The various types of headcrabs in Half-Life.
- The X Parasites in Metroid.
- In [PROTOTYPE], Alex Mercer gets infected with a virus-specific parasite, which debilitates him until he finds a cure.
- Las Plagas in Resident Evil 4.
- The Zerg Queens in StarCraft love this trope. They have two kinds of nasty parasites at their disposal, ones that crawl inside the host and feed information back to the Zerg, and another that grows inside a biological unit and then emerges Chest Burster fashion. There's also a subtype of both that infects an entire Terran command center, turning the humans inside into something between zerg and suicide fanatics.
- Shin Megami Tensei III: Nocturne's Magatama are an odd case, being functionally either mutualistic or commensalistic (physically transforming their hosts into half-demons and granting them various bonuses and serving as a combination or armor and spellbook) but with parasitic traits and explicitly named as such (when leveling up, there's a good possibility of inflicting adverse effects on the user, strong demons are drawn to its power).
- Eve in Parasite Eve is a mutant mitochondria that evolved to kill its human host and took over her body while the host's body mutates beyond what it once was. Any creature within Eve's presence either goes up in flames or has its own mitochondria mutate and take over the host body. Aya's mitochondria evolved in response to counter Eve by granting Aya powers similar to Eve's without the Body Horror aspect, thus making Aya's relationship with her mitochondria a mutualistic one. In the sequel, Aya notes that her own mitochondria are keeping her body young because a youthful host is advantageous and while she doesn't exactly mind looking and feeling young, she worries that her powers scare people and believes she may not even age normally.
- The Big Tick in Ben 10, who is a parasite that eats planets. Unfortunately, the planet does not usually survive.
- Parodied with "Brain Slugs" in Futurama; unlike most versions of The Virus, which are typically capable of a bit of subtlety, Brain Slugs are blatantly visibly attached to the infectee, and make statements such as "Your mission for today is to go to the Brain Slug planet and stand around without wearing hats", in a stilted monotone.
- Ming's "mutant life-form" in the Defenders of the Earth episode "Terror in Time" is another example. It takes the form of an amorphous purple ooze which the Phantom describes as "a parasite that feeds on living flesh", seconds after Mandrake has been attacked by the mutant. Mandrake spends most of the episode with part of the mutant attached to his arm and, while his life is never in any immediate danger - his magical powers also seem unaffected - it is clear that the mutant is weakening him; he struggles to climb the cliff to Warlock's castle even with Lothar's help.
- And in case you were wondering by this point, yes, you have these as well, although generally a lot less than the mutualistic/commencialistic symbiotes as your immune system tends to wipe out the parasitic ones before they become too uppity. Most infectious diseases are caused by parasitic symbiotes and the immune response to them.
- [[http://en.wikipedia.org/wiki/Toxoplasmosis Toxoplasma gondii. Parasites that mostly live in mice and rats, but require a cat's body to reproduce, therefore they mess with the rodents' brain chemistry to make them easier for cats to hunt so they will get eaten, and the parasites can then reproduce within the cat's intestines before being... excreted. They can survive in just about any warm-blooded creature, humans included, but whether or not they are actually harmful to humans is a debated topic, and research is on-going to what (if any) are the effects on an infected human.