Xenomorphs (Aliens/Alien vs. Predator) can convert humans into Eggs for their spawn...
...and primarily reproduce by infecting other beings such as humans with special tailored cancers...
...which absorb genetic material from the host that form into a Chestburster.
Xenocytes (Ben 10) transform those they attach to into DNAliens.
The DomZ (Beyond Good & Evil)
A member of Alpha Section (Beyond Good & Evil) has been infected by the DomZ.
Kos Parasite (Bloodborne)
Daemites (BloodRayne) can infect humans and wrap around their spinal cord...
...allowing them to control the host's body.
Revenants (Code Vein) are turned into vampiric creatures by the addition of Bio Organic Regenerative parasites to their hearts.
Once a charter member of The Green's parliament, Matango (DC Comics) was corrupted by the Grey Force. Now an avatar of fungal life, he can reach and infect/convert/dissipate any other life-force with his yeast based power.
Nameless Worms (Deadman Wonderland) are parasitic worms that infect humans, turning them into Deadmen
Nest Keepers (Dishonored 2) have eggs lain in their bodies by the bloodfly parasites, forcing the victims to protect the nests even with the loss of limbs, ultimately releasing a warm of bloodflies upon death.
Baby (Dragon Ball GT) can infect other beings with Tuffle Parasites, which he does in an effort to recreate the Tuffle species.
A Tuffle Parasite (Dragon Ball GT) within Majin Buu's body.
Vegeta, Trunks, Bulla, Gohan, and Goten (Dragon Ball GT) have all been infected with Tuffle Parasites.
After choosing Vegeta as his permanent host, Baby (Dragon Ball GT) mutated his body, converting him into a Saiyan-Tuffle hybrid.
The Risen (Fire Emblem series) are corpses inhabited and reanimated by Thanatophages, parasitic insects created by the alchemist Forneus.
Formers are a variety of humans that have been thoroughly infested with Imulsion (Gears of War series), a planetary parasite.
Megaera's Parasites (God of War: Ascension) can infect other beings, converting them into insectoid creatures under Megaera's control.
Zombies (Hλlf-Life series) are formed when a Headcrab attaches itself to a victim's head, leaving them conscious of their actions but unable to stop them.
The Flood (Halo series) infest others with their spores or Infection Forms, transforming them into pseudo-zombies.
Sadu-Hem (Hellboy) is a parasitic fungus that can infect humans...
...turning them into frog monsters.
The Cordyceps Brain Infection (The Last of Us) is a parasitic fungus affliction which seeds into the infected host turning them into hideous monstrosities over time.
Clickers are members of the Infected (The Last of Us) that have had the fungal infection overtake the majority of their heads, granting them increased durability in that area, but also depriving them of their sight.
Symbiotes (Marvel Comics) are parasitic entities regularly bond with other entities for survival, allowing them to recall information from previous hosts...
...while attaching traces of themselves to said hosts' DNA as a way of sharing information with the rest of their hive.
Cletus Kasady/Carnage (Marvel Comics) can infect other beings with portions of his symbiote, allowing him to control them.
Poisons (Marvel Comics) parasitize the host of the Klyntar to which they bond, transforming them into yet another Poison.
The various transmetal aliens (Marvel Comics) in fact use a parasite in order to convert and feed upon whomever they need too....
for it contains their consciousness which can be spread around and encode their essence upon a host beyond the effect of mere leeching.
Vocal cord parasites (Metal Gear Solid V: The Phantom Pain) are parasitic organisms that infest their hosts' vocal cords, eventually turning them into zombie-like creatures that are slaves to the parasites' will and only seek to spread their infection.
The Parasite Unit/Skulls (Metal Gear Solid V: The Phantom Pain) are essentially zombified by their parasite, enabling them to have a variety of special abilities, but little to no cognition.
Puppet soldiers (Metal Gear Solid V: The Phantom Pain), humans infected by parasites emitted by the Parasite Unit.
The "one that covers" (Metal Gear Solid V: The Phantom Pain) are parasitic organisms that can physically enhance and transform the host, coming in Mist
Quiet (Metal Gear Solid V: The Phantom Pain) was infected by a variety of the "one that covers", granting her a variety of superhuman abilities.
The X Parasite (Metroid Fusion)
Las Plagas (Resident Evil series)
The Queen Leech (Resident Evil 0) after merging with the corpse of James Marcus.
The Nemesis T-Type (Resident Evil 3: Nemesis) was created through the use of the NE-α Type parasite in high quantities.
The Chainsaw Man, infected with a Plaga (Resident Evil 4).
Osmund Saddler (Resident Evil 4) infested himself with a dominant species Plaga to control other Plagas.
Ramon Salazar (Resident Evil 4) fused with the Queen Plaga.
A Type-2 Plaga (Resident Evil 5) infecting someone to transform them into a Majini.
E-001, a.k.a. Eveline (Resident Evil 7: biohazard) is an E-Type who can manipulate mutagenic mold to mutate and corrupt others.
The Baker family of Jack, Marguerite, and Lucas were all mutated into horrific beings in body and mind by Eveline's mold (Resident Evil 7: biohazard).
A Green Zombie (Resident Evil: Outbreak) infested with a t-Virus infected plant.
An NE-β Type Parasite (Resident Evil: Operation Raccoon City) has infested a zombie, turning it into a Parasite Zombie.
Oozesquitos (Rise of the Teenage Mutant Ninja Turtles) are mutant mosquitos who mutate humans through their bite.
The abersyn symbiote hive of the Queen of Ktath'atn (Star Wars) turned its hosts into vampire-like entities, with the leader of the hive feeding off of subordinates' life force.
An Infested Terran (StarCraft)
Alexei Stukov (StarCraft)
Void Zombies (Starfinder) are infested with larval akata.
The Strigoi vampires (The Strain) are created through a worm that acts as both a parasite and diffuses a pathogen
Chimera Animas (Tokyo Mew Mew) are blob-like alien parasites that infect animals and change them into monsters.
Makai Insects (Yu Yu Hakusho) are parasitic demon bugs that infest humans, feeding on the evil in their hearts...
...and turning them into violent, zombie-like Possessed Humans.