The Symbiote

A "Bugsucker" is a symbiotic creature, combining Bugbear (host) with Stirge (guest)

"Of all the parasites I've had over the years, these worms are among the — hell, they are the best!"

— Philip J. Fry, Futurama

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:

• 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.

In fiction, all types are very frequent guests in Science Fiction and Fantasy. The Heroic Host specifically gets their powers from such an arrangement, though the specifics vary.

Compare Puppeteer Parasite (parasites that control other organism's brains) and Face Full of Alien Wing-Wong (parasites who use our bodies for reproduction), which could be subtropes of this. Also compare Chest Burster. See also Heart Drive for a similar phenomenon involving biological Soul Jars.

For details on hosts for symbiotes, see Body and Host.

Symbiotic Possession and Translator Microbes are versions of Mutualism that are particularly common in Speculative Fiction.

When The Symbiote covers the host's body as some sort of costume, whether for good or bad, it's almost invariably a Clingy Costume. Supetrope to The Infested, which covers cases where a large swarm of tiny creatures makes its home inside the body of a bigger one.

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Examples of Mutualism:

Anime and Manga

• Chapter 122 of Attack on Titan indicates that the true origin of all Titans was actually a worm-like creature (resembling Hallucigenia) that attached itself to the spine of a slave girl who literally stumbled upon it by complete accident. This organism and its ability to turn people into Titans was propagated over centuries by consuming the spinal fluid of the girl and her descendants.
• 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.
• 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 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.
• In Pokémon: The Series, a Shellder latches onto a Slowpoke permanently to create a Slowbro or Slowking.^note This is not true in the games; Slowpoke evolves sans Shellder. Professor Westwood V hypothesizes that Slowpoke evolving into Slowbro via Shellder clamp is mutually beneficial to both Pokémon, as the now-Slowbro is able to use its forearms freely because Shellder acts as a counterweight while Shellder is now able to travel on land.
• An spiritual, magical version appears in Yu-Gi-Oh! Marik, in order to fight against his own Superpowered Evil Side, bonds with Yami Bakura and gives him important knowledge, like how to use Ra.
  • Another example is Yami Yugi / The Pharaoh, who was in the Millenium Puzzle and in exchange for having a body (and friends) he will play children's card games.

Comic Books

• The DCU:
  • Green Lantern: The color entities 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.
  • Wonder Woman (1942): Ambulatory Rykornian males live in the husks they were born from on their stationery birthing stalks, and both types of Rykornian works to protect the other.
• Marvel Universe:
  • 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 Incredible Hulk: In the "Crossroads" arc, the Hulk goes to a planet where the food is poisonous to animals unless they have a symbiote attached.
  • Spider-Man: The Symbiotes, 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.
    • After losing her powers, Spider-Gwen joins with her universe's Venom symbiote to regain them. She and her symbiote eventually enter a healthy relationship after Gwen works through her anger issues (her father had been wrongfully jailed and she's been forced to work for the Kingpin since then, leaving her with a lot of pent up rage) and is able to teach her partner more positive emotions. The only major issue she has come across with her "gummy spiders" since is learning to keep them properly fed.
    • The precursor and prototype of the Symbiotes was All-Black the Necrosword. All-Black was originally the shadow of a primordial god of darkness, Knull which he split off. After Knull lost All-Black in a battle, he would go on to create the entire Symbiote race as a substitute for his lost weapon. All-Black has been wielded by others afterwards, but it still ultimately serves Knull's desire to wipe out all life in the universe (especially other gods). Hence why it acts as The Corrupter to its "masters" like Gorr (enabling and amplifying his misotheism) and Galactus (from Devourer to Butcher).
• Spawn: Spawn's distinctive costume (which is standard issue for all Hellspawn) is actually a living, sentient demon in its own right, and possibly inspired by the Spider-Man examples below. 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 series 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.
• Wildstar: Several were created in the future. They grant their host bodies certain powers, but in exchange, they are allowed to feed off of them.

Fan Works

• Equestria: Across the Multiverse has 'Size Symbiotic Equestria' as it comes to be known. As the name suggests, this world is home to two species of ponies, Giants and the other Little Ponies who are tiny even by mainline Equestrian standards who live in symbiosis with one another. The Giants use their physical strength and size to do the heavy lifting and protecting their smaller partners, while the Little Ponies do the finer tasks and task in small spaces the Giants can't do or enter. It's stated they bond when young on a magical level and the death of one would have severe emotional effects on the other (though they can form another bond eventually). Also, all video games on this world are co-op for this reason.
• A Moth to a Flame: As opposed to the Grand Theft Me it pulls in canon, her being a more willing partner makes The Core decide to take this approach with Marcy. Rather than take over her body and assimilate her mind, it instead merges with her in a more equal way, allowing her to stay "at the helm", as Andrias puts it.
• 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 are much better off than those who act as parasites. In Dark World, 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.
• White Sheep (RWBY):
  • Jaune and his sisters all have Grimm "parasites" implanted in their bodies. Despite their names, the parasites don't weaken or control them at all, merely acting as advisers and translators for the Grimm. They also produce and control the Combat Tentacles that Jaune and the others are infamous for. The children can do all these things on their own, or even wrest control of the tentacles away from the parasite, but it is easier to let the parasites handle it.
  • Cinder has a parasite as well; in fact, the parasite she has in canon appears to be the inspiration for the parasites in the fic. She only mentions it once, however, and it never speaks, so it is unclear if it is the same type of parasite that Jaune and his sisters have.
  • When Yang's arm is cut off by Adam, Jaune implants a parasite in her that will be able to act as a replacement arm and bring her up to speed on what happened while she was unconscious. The parasite quickly proclaims himself Yang's son, Yang names him Yin, and Yang goes off to strangle Jaune for impregnating her and leaving her with the kid. Despite her complaints, she does quickly come to care for Yin, and she is able to adapt her fighting style around having an arm that she doesn't control but can burst into razor-sharp flailing tentacles.
  • All parasites shown also have the ability to go to sleep on command. Quite useful if you're doing something embarrassing you'd rather not have them listen in on, such as using the bathroom.

Films — Live-Action

• 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.
• Sputnik. Establishing the nature of the alien inhabiting the cosmonaut is a Driving Question for the protagonist Dr. Tatyana Klimova. It's initially assumed to be parasitic, but she eventually realizes they're symbiotic with a shared consciousness. The host acts as a terrestrial spacesuit for the alien, while the alien repairs any injuries the host might have.
• Star Wars:
  • The Tusken Raiders of Tatooine 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 banthas 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:
  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.
• Tremors 5: Bloodlines: The prehensile tongues inside the Graboids' mouth are, at least for the African subspecies, indicated to actually be separate organisms that live in symbiosis with the Sand Worms when one of them jumps out of his big brother's mouth and continues pursuing the heroes like some sort of killer land-eel.

Literature

• The Apocalypse Troll: The symbiote 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 Behemoth: The nameless worm-thing living inside Roger Harding 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.
• Crystal Singer: The symbiote has a low success rate for adaptation to human hosts. Those who survive, though, gain a Healing Factor that gives them enhanced physical strength and senses and a life span of centuries. The catch being that, once adapted, they can't stay away from the symbiote's home planet of Ballybran for long, before the symbiont starts to weaken and die. Crystal Singers (those who made the most complete bonding) can stay away for years. Some with lesser adaptations can't leave at all.
• Downward to the Earth: Sliders are amorphous organisms that live by attaching themselves to large animals, using them as transport in exchange for consuming the sweat and dirt on their skin. When Gundersen meets Seena after the latter has spent several years living alone at the former Niagara station, she has taken to wearing one as a form of clothing.
• Dragonback: The K'da is 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.
• Eden Green: The alien needle symbiote is mutualist: the needles are able to spread to new life forms, and the hosts gain nigh-immortality.
• The Edge Chronicles: One of the Deepwoods' native dangers is the aggressively carnivorous bloodoak tree. Bloodoaks, however, don't have any means of capturing prey — in practice, a bloodoak's just a trunk with a stomach in it. Each bloodoak grows alongside another plant, the tarryvine, which roots itself in its trunk; the tarryvine is mobile, agile and quite capable in a fight, but lacks a way to digest prey. Their relationship is straightforward enough — the tarryvine darts into the surrounding forest to snatch passing animals or people and dumps them down the bloodoak's mouth, creating a shared meal for the two symbiotes.
• Eight Worlds: 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.
• Evolution:
  • In "The Kingdom of the Rats", the borametz trees have adapted to live symbiotically alongside colonies of ants, termites and eusocial posthumans, which stores their seeds and plant new trees when conditions are good in exchange for feeding on the trees' sap. The narration notes that this will prove to be a very efficient system, and that the borametz trees will eventually outcompete most other plants.
  • The inhabitants of New Pangaea took the principle of cooperation and sharing to its extremes to survive in the dying Earth. The descendant of the borametz tree cannot survive without the insects that bring nutrients to its roots, and the mammals who bring it water, food, and salt, and plant its seeds. Even its leaves belong to another plant. Likewise, the symbiotes cannot have survived without the tree, which shelters them from predators and climate, and feeds them with its sap.
• Falling With Folded Wings: Olivia theorizes that the System is a parasite, providing energy to intelligent species so that they can grow stronger and then it can receive that energy in turn. While it does seem to be at least a bit mutually beneficial, the Elder Races complain that they were doing fine before the System started encroaching on their territory.
• 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.
• 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.
• 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 Hitchhiker's Guide to the Galaxy Trilogy's Babel Fish, a living plot device as a Symbiote.
• Honor Harrington: Treecats look something like six-legged house cats. They are essentially Bond Creatures, and removing one from their human results in both pining away.
• Hothouse:
  • The morel, a fungus which has survived into the future by evolving the ability to attach itself to other creatures to serve as a secondary brain, greatly increasing their intelligence in exchange for food and protection. It's quite common in the Nomansland, where almost every sort of creature — from giant termites to predatory trees — can be seen bonded to a morel. Beyond that, as the morel discovers by exploring a human host's Genetic Memory, modern-day humans lived in symbiosis with a similar fungus as well, one adapted to live within their brains, and the downfall of human civilization came about when increased solar radiation killed off their fungal symbiotes and robbed them of most of their intellect.
  • On the Earth's dark side, some surviving animals have come to depend on each to survive. A species of bat, for instance, has taken to ferrying spider eggs to high mountains that get just enough sunlight to warm the eggs into hatching, and then carries the spiderlings back; in return, the spiders weave a secondary net that they dip into bodies of water, catching fish for the bats to eat.
• Necroscope: The vampire-fungi technically fit here. The fungal leech gets a host with sentience and opposable thumbs; the host gets shapeshifting, enhanced psychic powers, enhanced strength, enhanced senses, and a craving for fresh blood and human flesh...
• 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.
• 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 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.
• 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.
• The Stormlight Archive: The Nahel bond between human and spren (essentially a sapient 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.
  • Certain animals can form bonds with lesser non-sapient spren, which is part of what allows the huge crustaceans collectively called greatshells to exist (the spren effectively makes the creatures lighter). It's not entirely clear how this affects the spren.
• 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.
• 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 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.
• Xandri Corelel: The planet Song is home to enormous, sapient sea creatures called the Voices. Each Voice has between two and a dozen Hands, smaller cephalopod-like creatures that follow them around their whole lives.

Live-Action TV

• Babylon 5: The Vindrizi from "Exogenesis". 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.
• Doctor Who: The relationship between a TARDIS and its Time Lord pilot is all but stated to be symbiotic. The TARDIS gets taken to places where it can 'refuel' and gets maintained, and the pilot gets transportation and shelter in exchange. However, while a Time Lord can survive without a TARDIS, a TARDIS generally needs someone to keep it in working order.
• Earth: Final Conflict: The Skrill are millipede-looking creatures genetically engineered by the Taelons. They burrow into their host's forearm where they feed off the body's energy. In exchange, the Skrill allows the hosts to fire powerful energy blasts.
• 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 relinquish 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.
• Star Trek: Deep Space Nine: The Trill. 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.

Tabletop Games

• Dungeons & Dragons has a few, but not for the faint of heart:
  • The third edition Fiend Folio had a section in the back of the book about symbiotic monsters. It introduced seven different new creatures that can bond with a host and give them benefits, although they can also try to take over a host like a Puppeteer Parasite if they have a disagreement.
    • The Cerebral Hood is a psychic parasite that attaches itself to a host by stunning it with a mental blast, engulfing its victim's head with its ray-like body, and jamming its tail down the victim's throat to meld with its nervous, digestive and circulatory systems. It drains enough Intelligence from its host to impose a -1 penalty, but on the upside, the host gets access to the Cerebral Hood's mind blast, keen senses, and becomes immune to inhaled toxins or diseases. Just, you know, there's a psychic stingray wrapped around the host's head now.
    • The Mind Leech is a parasite that burrows into a person's body and wraps around their spinal cord. Outside of a host they have no powers at all except for being able to communicate telepathically, but when bonded they get several other psychic abilities, although using them temporarily damages the host's intelligence.
    • The Psionic Sinew is a worm-like creature that a person can attach to their arm. It enhances the user's strength, generates a protective field around them, and can turn the host's hand into a powerful claw or give them temporary hitpoints at the cost of causing temporary dexterity damage.
    • A Fiendish Familiar is an evil face that can be grafted onto a person's body. It improves the host's spellcasting abilities and shares its knowledge of magic with them.
    • The Gutworm is more or less a fiendish tapeworm from the abyss that swims around its host's intestinal tract. Its host must eat and drink twice as much as normal and becomes fatigued more easily, but the Gutworm grants a passive bonus to their Constitution score, can freely cause them to fly into a Barbarian's rage, and neutralize poisons its host may fall victim to.
    • Soul Ticks drain blood like their mundane cousins, and may overfeed to punish a host who displeases them. But they also enhance evil or necromantic spells, and provide a constant protection from good effect for their hosts.
    • The Ghostly Visage is a ghostly undead that shields the host against mental effects and can manifest a terrifying face on the host's body.
  • More symbiotic creatures with rules similar to the ones from the Fiend Folio were introduced in the Eberron books. In the Eberron setting, symbionts are living weapons engineered by the Daelkyr.
  • A less morbid example is the Vizier's Turban. This creature can change its appearance to some degree, but always appears as some manner of turban, scarf or veil with a pair of gemstone "eyes." When bonded with a wizard, the Turban grants extra spell slots and magical resistance based on how many hit points its wearer is willing to transfer over in exchange. A Vizier's Turban can communicate telepathically with its wearer, and tends to adjust its own personality and outlook to match its host's Character Alignment. The Turban likes to engage in philosophical conversations and see new places, and becomes depressed should its host leave it behind, but despite its intelligence it isn't a reliable adventuring partner — a Vizier's Turban reacts with a sense of wonder to just about anything, so it won't think to warn its wearer if it sees a monster about to pounce on them.
  • Glimmerskins are incorporeal beings of pure Life Energy from the Positive Energy Plane. They yearn to experience the thrill of combat, and so commonly travel to the Material Plane to find mortal hosts to bond with for the course of a battle or adventure. The glimmerskin, which usually takes the form of a radiant suit of armor or glowing hooded cloak, splits any Experience Points earned with its host, while said host gets some combat bonuses, and enjoys constant exposure to the glimmerskin's healing touch. In fact, even if a host is at full hit points, the glimmerskin will "overheal" them with temporary hit points... with the side effect of potentially having the excess life energy explode out of the host in a 20-foot radius. Glimmerskins tend to not mention this when offering to bond with a host. Another complication is that the glimmerskin can have their own idea of how their host should be fighting, and should said host ignore their telepathic suggestions, the glimmerskin might covertly contact another prospective host and ditch their previous "wearer" in the middle of a fight.
  • But it's the Symbiotic Creature template from Savage Species that offers the most comprehensive version of this trope. The template combines two creatures into a single one, with the mental stats of the guest, the physical stats and HD of the host, and the skills, special abilities, and attacks of both. Because the cost of the template doesn't scale with the number/quality of features gained, it's one of the most abusable character creation options in the edition, but it also allows for some truly unique and flavorful monsters and characters.
• Hunter: The Vigil: Most Cheiron Group implants involve simply splicing a patch of tissue or a new organ into a hunter's body. Some, however, involve the implantation of entire symbiotic organisms, such as a swarm of man-faced hornets that live in your arm and attack things that you're angry at or a worm wound around your heart that protects you from magic at the cost of making you more nihilistic.
• Rocket Age: The Fur Frog, a small creature that Venusians keep as pets. They 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.

Video Games

• City of Heroes: Peacebringers and Warshades are 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.
• Destiny: The Hive are parasitized by eldritch larvae-like worms that feed on violence and bloodshed. As long as the Hive fight and kill to feed their worms, they receive immortality and Reality Warper capabilities. But if they start losing more fights than they win, their worms get hungry, and start eating their hosts instead. And as individual Hive grow more powerful, the hunger of their worm gets harder and harder to sate. The gods of the Hive, despite being the oldest and strongest members of the species, were forced to create a tithe system drawing on every single other member in order to have enough death to satisfy their worms.
• Heroes of the Storm: This is Abathur's main ability. He can place a symbiote on any ally on the map. Abathur gains the ability to remotely damage enemies with his otherwise useless body, while the ally benefits from shielding, healing, extra protection, and other bonuses if talented.
• League of Legends: Kai'Sa, Daughter of the Void, is a human who was merged with a Voidborn symbiote, manifesting around her as a semi-abstract Bio-Armor. Their relationship is mostly hostile against her but often errs towards mutualism — the symbiote wants to consume everything, including Kai'Sa, but she's able to use the powers it grants her to stave off its hunger through hunting and "consuming" other Voidborn.
• Pokémon:
  • Slowbro is a mutually beneficial merger between a Slowpoke and a Shellder. The Slowpoke gains "inspiration" and increased psychic power and the Shellder feeds of the sweet nectar from the Slowpokes tail.^note If you're wondering why the Shellder attached to Slowbro looks nothing like a Shellder otherwise, it's because it's actually an unusued Pokemon named Turban.
  • Pokémon Scarlet and Violet introduces Dondozo and Tatsugiri, a giant fish Pokemon and a tiny worm-like Pokemon respectively. Tatsugiri hides in Dondozo's mouth, where it'll act like a helpless worm to unsuspecting Pokemon, and when the prey comes close, Dondozo goes in for the kill. They then share their meals. Tatsugiri even has different forms for luring in different types of prey. This is even used in gameplay, with Tatsugiri's signature ability "Commander", which, when paired with Dondozo, raises Dondozo's stats. Dondozo's signature move is "Order Up", which has Dondozo make an elegant pose and then strikes them down when they least expect it. If a Tatsugiri is in its mouth, it raises one of its stats.
• The Secret World: Players begin the game by swallowing one of the Bees of Agartha and permanently bonding with it; as a result of this, the player character is empowered with magical abilities, Resurrective Immortality, and sporadic access to the Buzzing. However, in return for this service, the players have to serve as agents of the Bees' creator, Gaia — essentially acting as white blood cells for her body in times of crisis.
• Sigma Star Saga: The Krill Parasites are a major part of the Krill's Organic Technology. They envelop the host as armor, form a psychic bond with the host that improves performance, and provide a control link to the Krill ships. They're even part of the culture: the Parasites of female Krill grow wings that can be be detached and given to someone else as a sign of affection.
• 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.
  • 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 two of them have been together for so long that their personalities have effectively merged and they no longer consider themselves separate individuals.
• Stellaris: One event chain concerns a world that contains both nonsentient lizards and weird slug creatures with highly-developed nervous systems. After one of your scientists theorizes that the slugs are symbionts and volunteers to become host for one, the slug telepathically explains that they are a species of long-lived hive minds that used to have a mutually beneficial relationship with the reptilian species on their homeworld, until the reptilians grew suspicious and cast out the brain slugs, only to devolve into nonsapience. The brain slugs seek new hosts, and offer to join your empire. If you accept them, some of your Pops and leaders will gain the "Brain Slug Host" modifier which provides various bonuses, though at the cost of a 25% slower growth rate, and any time a symbiote pop appears on a planet, it will incur a -20 stability penalty due to suspicious locals until 20 years have passed as those without symbiotes get used to living with them.
• Subnautica: The main life forms of the Jellyshroom Caves ecosystem have this sort of arrangement. The fanged, eel-like Crabsnakes consume any herbivores that might threaten the enormous Jellyshrooms, which feed on the leftovers of the Crabsnakes' meals. In return, the Jellyshrooms' hollow stalks provide a hiding spot the Crabsnakes can use to ambush prey, or to keep their eggs safe.

Webcomics

• 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.
• The Gnoph in Gnoph live inside human lungs, and grant their hosts a variety of superhuman abilities.
• In Schlock Mercenary Fobott'r all have mohawk-style hair in a variety of colors. It turns out that this isn't actually hair but a symbiotic colony that provides the Fobott'r with blood filtration and neurochemicals while receiving protection and food in return.
• 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.

Western Animation

• The Worms in the "Parasites Lost" episode of Futurama. They build a society in Fry's body, and in return give him Super-Strength, a Healing Factor, and improve his mind (among other things). Unfortunately, Fry has to get rid of them to prove whether Leela fell in love with him or the man the worms made him into.
• Symbiotes in Men in Black: The Series. Basically, they're metamorphic heads which have to attach to other creatures to survive. Having one attached to you means that you get 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.
• 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.
• Transformers:
  • 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.

Real Life

• 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 Eukaryota (yes, humans are Eukaryota, too), playing an important role in the metabolism.
• Plants have both mitochondria and 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 a product of a symbiotic relationship in the first place (that is, they already were before accepting mitochondria into the 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.
• Acacia trees provide both nutrition and habitats to ants in the form of nectaries and hollow thorns, encouraging ants to colonize their branches. The ants receive a secure home and food supply, while providing the trees with a biting, acid-stinging defense against browsing herbivores.

Examples of Commensalism:

Anime and Manga

• The Guyver units from Guyver.

Comic Books

• 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.

Tabletop Games

• 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. The homunculus constantly whispers advice to its host and support them with their arcane abilities as the Curseling continues their search for further hidden knowledge.

Video Games

• Knights of the Old Republic II: The Sith Lords: 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 Pokémon Joltik feeds off other Electric-types' static electricity and stores it, as they can't produce their own. New Pokémon Snap sees this process in action when one can be lured to stick onto a Jolteon for a meal. It's implied that while the process is annoying for the hosts, they have plenty of electricity to spare once the tiny Pokémon is satisfied.
• 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.

Real Life

• 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.
• A bird nesting in a tree can usually be considered this. Depending on their diet, however, they may be mutualistic by consuming the fruit of the tree and aiding reproduction by spreading its seed with their excrements, or by consuming insects that might otherwise damage the tree.
• 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).
• Crows and ravens figured out a way to crack open nuts with minimal effort. They go find an intersection where cars are driving by, wait for an opening, drop the nut off in the path of a vehicle, and when the person drives over the nut, it cracks the shell. They may have figured out other ways to benefit from our otherwise mundane actions without bothering us.
• At least one species of tiny marine open-water crab has overcome the challenge of traversing long distances through predator-rich waters: they huddle within the gaps between sea turtles' shells and tails, nibbling stray tidbits from the turtles' feeding, and sometimes even breeding as they cling to their reptilian transport.

Examples of Parasitism:

Anime and Manga

• Parasyte: The Parasitic Beasts 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).

Comic Books

• Spider-Man: The symbiotes 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.
• Star Wars: The Screaming Citadel: Abersyn symbiotes attach themselves to the nervous system of their hosts, overriding all motor and cerebral activity and taking complete control of their bodies and minds.

Films — Live-Action

• Alien: The Xenomorphs utilize human bodies to reproduce via Face Full of Alien Wing-Wong.
• Cloverfield: The giant crab monsters swarming across New York were initially parasites clinging to the big guy, and which got shaken loose during the chaos.
• Star Trek II: The Wrath of Khan has parasitic eels native to Ceti Alpha IV, which Khan uses to control several Star Fleet operatives.

Literature

• 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.
• Animorphs:
  • The Yeerks, who are like the Goa'uld but without benefits to the host (unless the host is The Quisling, in which case they get privileges not extended to involuntary hosts, like not being caged while their Yeerk is feeding). They generally forcibly take hosts, as in their normal states they're blind, powerless slugs. There is even a faction of Yeerk society that disagrees with the morality of taking hosts forcibly, and will only infest the willing, who act as an underground resistance movement.
  • The Iskoort are a species closely related to the Yeerks, except that these slugs learned how to bio-engineer braindead host bodies for themselves, and have even gone so far as to bioengineer themselves to be wholly dependent on those host bodies, such that they cannot live in any other kind of host. They were influenced by the Ellimist as a back-up plan in case the Yeerk Empire is not defeated: eventually the Yeerks would encounter the Iskoort and see that there is another possibility besides parasitism, and it is known that there is a fair chunk of the Yeerk population who would wholeheartedly embrace that alternative.
• The Cancer Cowboy Rides, by John Connolly: The Black Worm 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."
• Cradle Series: This is how the Bleeding Phoenix feeds. It sends out millions of "blood shadows" which latch onto a host, hollow them out from the inside, and carry that power back to the Phoenix. Nine out of ten hosts are little more than puppets, attacking everything and everyone blindly. The minority, however, gain control of their blood shadows and use it to become far more powerful than they ever could on their own. They usually still end up serving the Bleeding Phoenix, they just have their own goals besides sacrificing themselves for its power. Even more rarely, people can take the blood shadow's power as their own without serving the Phoenix at all. Yerin ends up in this last category, and furthermore she ends up becoming a Herald by fusing with her blood shadow entirely.
• Downward to the Earth: Belzagor is also home to a variety of parasites, such as a sponge-like creature that nurtures its larvae inside living animals or a crystalline organism that infects cuts and spreads, fungus-like, itself within its host.
• Infected: The parasites are extraterrestrial 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.
• 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.
• Side Jobs: In the short story Day Off, two of the Alphas are afflicted with "supernatural fleas".
• Tress of the Emerald Sea has the spore-eaters, who are infected with a special parasitic form of one of the twelve aethers. The aether parasite is constantly leeching water from the host's body to send back to the prime aether, though it will use its powers to protect its host from other threats so that it can keep draining them.

Live-Action TV

• 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.
• 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 8. 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.)
• Star Trek: The Next Generation: The original appearance of the Trill in the episode "The Host" are an example due to Early-Installment Weirdness before getting reworked for Star Trek: Deep Space Nine. They're called "parasites" and completely submerge the personality of their host body. It's even implied that they breed a humanoid species specifically to serve as hosts, living as blank slates until a Trill parasite is ready to take them over.
• 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.

Video Games

• Dead Rising: The "Zombees". 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.
• Half-Life: The various types of headcrabs latch onto host bodies and control them as zombie-like creatures.
• Parasite Eve: 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.
• Pokémon Sun and Moon: The Ultra Beast Nihilego attaches itself to other beings, not directly taking them over but instead driving them mad. Its code name is Symbiont (Parasite in the Japanese version). The Pokedex flavor text mentions its victims becoming violent via its neurotoxin, which is shown when it takes over Lusamine.
• [PROTOTYPE]: Alex Mercer is infected with a virus-specific parasite, which debilitates him until he finds a cure.
• 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).
• StarCraft: The Zerg Queens have two kinds of nasty parasites at their disposal, ones that crawl inside the host and feed information back to the Zerg and others that grow inside a biological unit and emerge ChestBurster-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.

Web Videos

• Vaguely Recalling JoJo: Like the original manga, Empress is a parasitic Stand trying to devour Joseph Joestar's body. To make things worse, it is immune to Hamon because it is part of Joseph's body. Joseph defeats it with the help of Ceasar Zeppeli's ghost.

Western Animation

• Ben 10: The Big Tick is a parasite that eats planets. Unfortunately, the planet does not usually survive.
• Defenders of the Earth: Ming's "mutant life-form" in "Terror in Time" 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.
• Futurama: Parodied with the Puppeteer Parasites known as Brain Slugs. 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.

Real Life

• And in case you were wondering by this point, yes, you have these as well, although generally a lot less than the mutualistic/commensalistic 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.
• 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.

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