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The Souls in Stephenie Meyer's "The Host" are alien parasites that assimilate other races by being placed onto a host's spinal column and erasing the old personality. They are utterly peaceful and never lie, fight, argue, and completely trust one another.
The Comprise, a software-mediated human hive-mind that has taken over Earth and is in a state of cold war with the rest of the human race, in Michael Swanwick's Vacuum Flowers.
The Conjoiners, a race of cybernetic humans who share thoughts electronically and are dedicated to improving themselves through increasingly advanced technology. From Alastair Reynolds's Revelation Space series of novels.
The Culture series by Iain Banks mention assimilating life forms ("Aggressive Hegemonizing Swarms"), also those that are gladly assimilated. The Culture itself assimilates other cultures, but not by force.
When characters in Charlie and the Great Glass Elevator visit Minusland, Willy Wonka warns them all about Gnoolies, invisible creatures that are harmless to humans unless they bite, in which case the bitten victim goes through a slow, painful process of becoming more Gnoolies.
Jarts, an alien race devoted to archiving all life and information in Greg Bear's novels set in The Way multiverse.
Killiks, a fictional race from the Star Wars universe
Gaia in the latter Foundation novels of Isaac Asimov, an entity which unified all atoms of matter into a collective consciousness for the benefit of all.
Starro the Conqueror in the DC Comics universe. Can release spores to take mental control of populations and heroes.
The Electric Church, a pseudo-religious organization made up of robotic "monks" with human brains in The Electric Church by Jeff Somers.
The fungus-like lifeform that Richie Grenadine has been transformed into after drinking a contaminated beer in Stephen King's short story "Gray Matter"
The unnamed alien race whose artifacts are discovered by the people of fictional Haven, Maine, in King's The Tommyknockers. None of them are alive, but use of the alien technology gradually turns the users into beings more and more like its creators.
The Borg are a race of cyborgs in the Star Trekuniverse. They are known within and beyond Star Trek fandom for their relentless pursuit of what they want to assimilate, their rapid adaptability to almost any defense, and their ability to continue functioning after what may seem a devastating or even fatal blow seemingly unaffected. As such, the Borg have become a powerful symbol in popular culture for any juggernaut against which "resistance is futile".
Neural parasite or "blue gill" from the Star Trek universe. Parasitic being which tried to infiltrate Starfleet by physically connecting with brains of high-ranking admirals within the organization (From TNG episode "Conspiracy").
In the later TNG episode "Identity Crisis", several Starfleet personnel who visited the planet Tarchannen III were unknowingly infected by a parasite planted by a member of an alien species indigenous to the planet; by the end of the episode several of them have irrevocably become members of that species.
The Cybermen, a cyborg race introduced in 1966 in the TV series Doctor Who, from Earth's twin planet Mondas. They take suitable human subjects and supplement or replace organic parts. Unlike the Borg, each Cyberman is an individual, but devoid of emotion or loyalty to their former life, with little to no chance for "reversion" to their original state. Barring experiments with cats and dogs out of desperation in "The Next Doctor" (2008), Cybermen are usually limited in their assimilation to converting similar creatures to them, principally humans from Earth. As of "Nightmare in Silver" (2013) however, Cybermen possess the ability to convert "any organic lifeform".
The Daleks, also from Doctor Who, are a race of mutants descended from the human-like alien race known as Kaleds. Despite being bent on universal conquest and the extermination of all non-Dalek life, they will occasionally assimilate useful non-Daleks into their ranks by various methods. In later series, they can partially Dalekise humans and similar-looking species through use of airborne microscopic nanobots.
Kobali from the Star Trek universe. They reproduce by reanimating the dead of other species and converting them into Kobali.
Also in Doctor Who, the episode School Reunion features the Krillitanes, bat-like creatures who absorb characteristics from other species which they have conquered.
Stargate SG-1'sGoa'uld, a race of parasites that enter the bodies of their hosts whom they then control, and Replicators, a mechanical race that absorbs technology and annihilates entire galaxies for resources.
The Invaders from Getter Robo: Armageddon, a race of extraterrestrial Invaders which have the power to possess humans, mutating them into hideous creatures, as well as survive even if reduced to tiny pieces. They can also assume any shape they want, as they are amorphous in nature. Their most distinctive features are their many eyeballs and protruding spikes.
The Black Oil or Purity from The X-Files which is a sentient virus which infects humans and uses their body to grow a new life-form or alternatively can simply take control of the infected person
The Espheni on Falling Skies often "harness" the young of other races with biological entities that mentally enslave them and gradually turn them into Espheni.
The Vindrizi in Babylon 5 are a race of parasitic/symbiotic aliens that overwrite the personalities of their hosts to help preserve the knowledge and experiences of previous hosts. This is usually done voluntarily, but removal of the creature from its host is permanent and can lead to death for both the host and Vindrizi.
Star Control 2 There is a race called the Orz. From Project6014 Dialog the Orz seem to change shape and appearance drastically depending on the atoms of whatever they "assimilated."
The Scurge of Scurge: Hive are a cellular assimilation organism that rapidly infect biological, mechanical, and even energy based systems. Anything thus absorbed has its intelligence (if there is any) transferred back to the Source, which is the spawner and collective hive mind of the Scurge virus. This allows the Source to reach sentience.
Aparoids, a race of bio-technological organisms that were the central antagonist of Star Fox: Assault that sought evolution through assimilation (referred to in the game as "Aparoideation"). They are capable of assimilating both machines and living organisms. This continues the Star Fox series' tradition of referencing other science fiction works.
The Omar are a race of cyborgs in the computer game Deus Ex: Invisible War. They assimilate humans into the Omar (though this is usually voluntary). The Omar are supposed to have a merged collective consciousness, though many Omar will refer to themselves as "I" instead of "We." This seems to show that the Omar still retain some individuality.
The Zerg from the game StarCraft are an insectoid race that assimilate (usually referred to in-game as "infesting") or destroy every race that they come into contact with. Their attempt to overpower and assimilate the Protoss forms a major part of the storyline of the game.
The Undead Scourge in the Warcraft universe is a massive army of living dead creatures residing on the continents of Lordaeron and the Eastern Kingdoms. They spread a plague of undeath that turns all it infects into Undead. They are under the absolute control of the leader of the scourge, the Lich King. They seek to destroy all life.
The Flood from the Halo: Combat Evolved line of video games is a parasitic life form consisting of an Infection Form that burrows into a life form's skin and takes control of their neural pathways, transforming them into a Flood Combat Form. The host is eventually made into a Carrier Form, to create more Infection forms. The Flood are capable of destroying all life and have only been stopped by the Halos.
In Half-Life 2 the Combine assimilate humans to Combine soldiers for their military, or workers called stalkers. The Combine's final goal is to assimilate all of Earth's population, drain their natural resources, and move on. Also appearing in Half-Life and Half-Life 2 are Headcrabs, a parasite resembling a large crab or tick. Headcrabs attach to the head of prey, mutating the subject and taking control of the subject's body, resulting in a Zombie-like entity called headcrab zombies. Headcrabs have been seen attached to humans and Combine.
In Resistance: Fall of Man, the Chimera, the game's main antagonists, are a race of unknown origin who perpetuate their species by assimilating humans. The Chimera infect humans with a transformative virus, which induces coma on human victims. Infected humans are then taken to "conversion centers", where they are transformed into different types of Chimeran creatures. The bigger Chimeran creatures are actually composed of multiple human bodies, making humans no more than "modelling clay" for the creation of new creatures.
The Beast from the game Homeworld: Cataclysm is a viral entity that infects starships and adds them to its hive-mind. The infection, spread via a "charged particle beam" or by simple physical contact, converts the ship's crew into a form of bio-circuitry which melds with the ship's system and is used to control it. This bio-circuitry is often visible on the outside of the ship's hull, lending the craft a ghoulish "zombified" appearance.
In the arcade game Defender and its sequel Stargate, Landers, the most common of the alien lifeforms the player must combat, attempt to abduct humans from the surface. If they succeed in reaching the top of the screen, they absorb the human's body and become Mutants, which then attack the player's ship with greater intensity and speed.
The Brains from the arcade game Robotron 2084 (like Defender a Williams game) turn humanoids into Progs, fast-moving agents of destruction that mercilessly home in on the player.
In Mass Effect, the Reapers are a race of synthetic life who convert the dead bodies of organic life into mechanical husks, though these husks are only used for psychological warfare against the organic species and the technology to do this is not their own.
"Agent USA" is a 1984 computer game designed to teach children spelling, US geography, time zones, and state capitals. The game was developed by Tom Snyder Productions and published by Scholastic. It was released for the Apple II, IBM PC, Commodore 64, and Atari 8-bit computers. A giant mutated television set known as the "Fuzzbomb" has begun infecting cities across America and turning people into walking balls of TV static. The player controls a secret agent assigned to defeat the Fuzzbomb and rescue fuzzed people.
Vampires in folklore and many fictional stories (including Buffy the Vampire Slayer and Anne Rice's Vampire Chronicles) increase their number by converting or "siring" humans into their own kind. This usually involves blood exchange between the vampire and the victim, and can be forced or voluntary.
Werewolves appear widely in games and films. They commonly increase their numbers by biting or scratching humans, which will turn them into werewolves, in a similar way to vampires.
Zombies appear in many horror films and stories (including Night of the Living Dead) and often increase their numbers through biting people. If the person is not killed, they will die within hours and rise up as a zombie themselves. Zombies usually do not consciously try to increase their number, however, as they seek to consume their victims. Thus, only those that are bitten and get away from or destroy the zombie that bit them are in danger of becoming a zombie, rather than a zombie meal.