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Parasitic Horror

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"As the writhing, teeming mass of mindworms swarmed over the outer perimeter, we saw the defenders recoil in horror. 'Stay calm! Use your flame guns!' shouted the commander, but to no avail. It is well known that the Mind Worm Boil uses psychic terror to paralyze its prey, and then carefully implants ravenous larvae into the brains of its still-conscious victims. Even with the best weapons, only the most disciplined troops can resist this horrific attack."
Lady Deirdre Skye, "Our Secret War", Sid Meier's Alpha Centauri

Parasites are organisms which depend on feeding off living hosts for their survival. For sapient creatures such as humans, this is an inherently horrific concept as it violates all our ideas about bodily autonomy and how we interact with other creatures. While "normal" predation is at least intelligible to a species of former hunter-gatherers, parasitism seems to us both pestilential and insidious, not an acceptable part of the circle of life but some abberant perversion of nature.

In fiction, this often takes on far more extreme forms, being infected by parasites the size of your fist, ones that take over all of your motor functions, or horribly mutate your body. It may be the offspring of some other lifeform implanted into you against your will, based off the similar behavior of parasitic wasps. Sometimes they will get inside your body through Orifice Invasion, otherwise they'll just burrow into your skin. In some cases the parasite might even be both intelligent and malevolent.

Sub-Trope of Body Horror.

Super-Trope to Puppeteer Parasite, Chest Burster, Parasite Zombie, Parasites Are Evil, Womb Horror, and The Infested.


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    Fan Works 
  • Abraxas (Hrodvitnon): Two of Ghidorah's "children" cause this. When the Many are being injected into their first human victims, it looks like something is slithering under the body's skin. One of the unborn Zmeyevich kicking like a regular baby against its mother's belly is just a little bit more visible than is human.

    Film — Live-Action 
  • The Bay is a Found Footage Film that starts out as a documentary of the Fourth of July celebration at Claridge, Maryland, only for the town to succumb to a plague of a mutated species of Cymothoa exigua spawned from the polluted bay. They infect the people through the water supply and eat them alive and spawn until they die, where they escape their host and jump to a new one. By the end of the film, nearly everyone in the film had died after the US government quarantined it, the footage leaked by the reporter that survived.
  • Growth is about an island that unwittingly hosts a secret government laboratory studying parasites. While they're intended to produce extremely big, shiny pearls, this is only a proof-of-concept. They are accidentally released, first infesting a creepy kid (who, does, as a matter of fact, have them swarming out of her eyes), then overrunning the whole island. The Cruel Twist Ending of the film shows that those pearls are actually their eggs, and they're hatching...
  • The Mummy Trilogy: Scarab beetles can quickly burrow into a person's skin and eat them from the inside out. They are especially fond of human brains.
  • The Ruins: The evil vine not only eats people (or drinks their blood), but it also infects them with spores which then proceed to grow inside the victim. One of the main characters ends up killing herself as she tries to cut them out.
  • Sputnik: The alien lives in the host's oesophagus and stomach, and secretes a toxin that knocks out the host and relaxes his muscles so the alien can exit via the mouth to hunt and feed, which it does every night. Once outside the body and ingesting oxygen it grows from a snakelike form to a multi-limbed Starfish Alien 1.5 metres in length. It's speculated that the alien is using the host as a spacesuit until it becomes fully adapted to Earth's environment, whereupon it will discard the host like a cocoon (turns out the relationship is symbiotic).
  • The Thing (both 1982 and 2011): The titular villain is an extraterrestrial single-celled organism that can infect its victims by mere touch and hides inside of them. If its cover is blown, the Thing creates a horrific pandemonium of meat, tentacles, and mouths out of its host's body. To hide and hunt efficiently, the Thing orchestrates paranoia to the heroes, causing them to accuse each other for being the alien menace.
  • Venom (2018) is a subversion of this. While Eddie is initially terrified by the Venom symbiote using his body as a host, and is especially horrified by his new diet of human brains, Venom is actually sentient and able to be reasoned with — and, as consolation, being his host also grants Eddie all kinds of cool superpowers. They grow to genuinely like and trust one another, to the point where Venom is hurt and angered by being called a parasite, insisting their relationship is mutually beneficial. By the end, Eddie's grown to like having Venom around, and offers the compromise that they only eat bad people, to which Venom agrees, allowing them to become a full-on Horrifying Hero.

  • All Tomorrows: On one world that resisted their invasion, the Qu punished the people living there by transforming some of them into an array of parasites. Some were tortoise-sized and ambulatory, others were fist-sized and lived attached to hosts, and there was even one variety that infested the wombs of its victims. Most of them went extinct after the Qu left, but one variety subverted this, as they went on to regain sentience and formed a symbiotic relationship with their hosts.
  • A Little Vice: The seeds that empower the Beasts are this, and push the hosts towards their desired behaviour.
  • Things Have Gotten Worse Since We Last Spoke: The last parts of the book involve Agnes being convinced by Zoe to consume raw insect-infested meat, willingly giving herself a tapeworm in order to fulfill her desire to be a mother.
  • The Troop involves a parasite-based bioweapon created as a side-gig to a genetically engineered tapeworm diet aid. While the diet aid was meant to become The Symbiote, the bioweapon took all the bad things about them and cranked them up, and added a few more besides.

    Live-Action TV 
  • Babylon 5: The episode "Exogenesis" features giant centipede-looking alien critters who are shown sinking into the flesh and bonding to the spinal columns of assorted lurkers in Downbelow, where they seem to take control of their hosts' bodies. Contrary to what Marcus and Doctor Franklin initially think, the Lurkers aren't victims, but volunteers, and the alien critters are actually symbionts, sharing their memories with the Lurkers in exchange for the ride. Basically what the Trills from Star Trek would be if the Federation didn't know about Trills.
  • One episode of Earth: Final Conflict introduces a parasitic worm that burrows into the body of its host, making them completely unafraid of everything, up to and including death. However, after an hour, the parasite will kill the host.
  • The Expanse: In season 4, Holden and Amos are trapped on an alien planet with the other colonists and have sought refuge in the ancient artifacts. It later turns out that the water contains parasites which nestle inside their eyes, slowly causing them to go blind.
  • In one episode of Primeval, a flock of dodos manages to escape into a building in the present day. The dodos themselves are about as dangerous as you might expect, but they bring with them a highly infectious worm-like parasitoid that can also transfer to humans.
  • Stranger Things: The Mind Flayer manages to leave a bit behind when it injures El, which starts wriggling and painfully crawling around under her skin later. Jonathan has to cut her leg open and try to dig it out with his fingers while they're all hiding from the Mind Flayer.

    Multiple Media 
  • Star Trek:
    • Star Trek II: The Wrath of Khan features Ceti Eels, which burrow into their victim's head through the ear canal and attach themselves to their host's brain. This has the side effect of making their victims extremely susceptible to suggestion. Khan uses this to hijack the starship Reliant and force Captain Tarrell and Commander Chekov to lure Enterprise into an ambush. When Khan orders Terrell to kill Kirk, he's able to resist the eel's influence just enough to eat his phaser before Khan can force him to murder the Admiral.
    • The Star Trek: The Next Generation episode "Conspiracy" plays the parasites for various types of horror: firstly, they change people's personalities, secondly, they make people violent and super-strong, thirdly they themselves look pretty gross with their appendages sticking out of the person's neck, and fourthly, some of them breed in somebody's stomach, causing him to explode.
  • Ultra Series:
    • Ultraman: The Next: Beast the One, the Big Bad of the film is a parasitic, space demon who merges with a human host and slowly overrides their mind, body and finally, kills them altogether, absorbing its host into itself. It can do the same with other living creatures for the sake of augmenting itself.
    • Ultraman Z: Celebro, Big Bad of the series, is a space entity that takes possession of various organisms to carry out his evil plans. His possessions are shown to be rather unsettling as his body language shows signs of twisting the bodies and actions of those possessed to suit his needs and his possessions are shown as rather painful.

    Tabletop Games 
  • Blackbirds RPG: Those who gain the attention of the Child of Longing or are otherwise touched by her influence will often be afflicted with a horrifying transformation simply called the Change. The afflicted suffer a variety of symptoms from voracious hunger to memory loss, but the most viscerally horrifying is the innumerable flesh-eating Caoimhe moths gestating within their bodies. At its final stage, the victim is reduced to little more than a human bomb filled with the moths, magically kept alive and conscious while being trapped in an agonizing existence of waiting to explode while feeling thousands of flesh-eating insects crawl through their body.
  • Dungeons & Dragons:
    • Illithids (a.k.a. Mind Flayers) are parasitoidic, meaning their young can only (properly) mature inside a living host (when they don't, the result is a Neothilid), and kill them when they "hatch". In this case, the larvae eat their hosts brains over the course of around a week, and replace everything south of the nose with tentacles. It gets more complicated when different races get involved, but the basic concept remains the same.
    • Slaan come in two varieties (red and blue), which are basically their sexes. They each lay eggs in an opponent on a successful unarmed melee touch attack, which need to be fertilized by the other color of slaan to take. In certain editions/settings, blue slaan eggs hatch into red slaan, and red slaan eggs hatch into blue slaan. In another, red slaan carry a virus that transforms the infected into blue slaan, who use the ovipositing claws.
  • Pathfinder:
    • Boregrubs are parasitic worms that infest rotten wood and leap out when they sense suitable hosts (i.e., player-characters). Boregrub-infested doors are a common booby-trap in Haunted House-themed dungeons.
    • A certain Cthulhu Mythos-themed module has Dark Young of Shub-Niggurath born this way. They provide the host with immunity to mind-control for a couple of days before exploding out of their heads in a welter of gore and tentacles.
  • Warhammer 40,000:
    • The tyranids have several abilities involving weaponized parasites:
      • Devourers are a Living Weapon analogous to a heavy machine gun, which fires nerve-eating maggots.
      • The Hero Unit "The Parasite of Mortex" has a special attack that causes infantry units killed by it to be replaced with a Ripper Swarm, representing them being devoured from the inside out by the ravenous, larval bioform.
      • Mind-control worms are available as wargear.
    • The chaos god Nurgle, being a reflection of people's feelings of acceptance towards the bad part of nature, holds sway over parasites among other illnesses.
      • One of his daemonic gifts is a Beneficial Disease in the form of a huge parasite that makes whatever vehicle it infests more healthy (although this technically makes it a symbiote, it's still a huge worm suckling on the vehicle's Machine Spirit).
      • Those infected by his plagues often end up infested with maggots, either mundane or daemonic, that are attracted by the stench of the victims rotting flesh.
      • The Nurgle Champion Typhon has a mutation called the "Destroyer Hive," which resembles a termite mound growing out of his back. At will, it can disgorge a swarm of daemonic wasps.

    Video Games 

    Web Originals 
  • A certain Creepypasta involves a man on a bet to lose weight take anabolic steroids and a tapeworm pill together. The tapeworm becomes giant and eats all his organs.
  • Don't Walk Home Alone After Dark: The titular being in The Worm is a psychic parasite; it can latch itself onto the psyche of anyone who knows it exists and torments them in their dreams, feeding off their fear and other negative emotions. It can be spread from person-to-person by having someone already infected tell someone else about it.
  • Deep Root Disease from Gemini Home Entertainment involves an alien "plant" growing on human hosts, spread by insectoid creatures called "Woodcrawlers." As it grows, it converts humans into "fake people," who are aggressive and territorial. It eventually reduces them to a tangle of red fibres in the vague shape of a person who Can Only Move the Eyes. This phase is actually a carniverous plant called "Nature's Mockery."
  • Mortasheen, a horror-themed Mon world heavily influenced by real-life biology, has too many parasite-based monsters to list individually. Wormbrains are Puppeteer Parasite flatworms that control genetically engineered host bodies. Botanical monsters are cultivated as parasites of natural plants. The Genetimorph and Under fiend are dangerous parasitoid creatures whose offspring are Chest Bursters, whereas the Wrigglegeist is an endoparasite that lives harmlessly inside an unwary host. Still more parasites feed off of weirder things like genetic material and neural activity.