After touching his living computer virus...
...Rick Twitler (Henry Danger) was able to turn people into living virus by sneezing on them
The Hōgyoku's (Bleach) ability to manifest the desires and potential of any creature allows it to, amongst others, turn Hollows into Arrancar, Shinigami into Hollows, and humans into Fullbringers.
Skinner Sweets (American Vampire) converting Pearl Jones into an American Vampire by feeding her his blood.
Deacon Frost (Blade) viewed Vampirism as a gift.
The Shroobs (Mario & Luigi: Partners in Time) convert other beings into Shroobs or Shroob-like beings via a process called "Shroobification."
Diagon (Ben 10) could convert countless beings into Esoterica, his cross dimensional foot soldiers.
Joseph Bertrand III (Infamous) converting a human into a Corrupted.
The Moon Pool (H2O: Just Add Water and Mako Mermaids) transforms any human into merpeople on a full moon.
Cybermen (Doctor Who) seek to convert every living thing in the universe into one of their race
Doctor Constantine (Doctor Who) being transformed into a gas mask zombie
Noah (Doctor Who) being transformed into a wirrn.
Using the Immortality Gate, The Master (Doctor Who: The End of Time) turned every human on earth into 6,727,949,338 clones of himself.
Saturnyns (Doctor Who) could covert other species into members of their own kind via blood transfusion.
Children being converted into Axons (Doctor Who: The Golden Ones)
Hawk Moth (Miraculous: Tales of Ladybug & Cat Noir), with the power of the Butterfly/Moth Miraculous, is able to transform others into supervillains, complete with their own arsenal of superpowers.
Darkblade (Miraculous: Tales of Ladybug & Cat Noir) has the power to convert humans into members of his army of knights.
Pharoah (Miraculous: Tales of Ladybug & Cat Noir) is able to channel the power of Anubis, converting humans around him into mummies.
Puppeteer (Miraculous: Tales of Ladybug & Cat Noir) has the power to control those who's caricature she possesses, using Marinette's dolls as a means of doing so. When using this power on purified supervillains, the villains regain their powers and villain identities.
Gamer (Miraculous: Tales of Ladybug & Cat Noir) is able to convert humans into units of power, upgrading his mech with them.
Lilim (Monster Girl Encyclopedia) Are an enhanced race of succubus who are hailed as daughters of the Demon Lord; like their weaker counterparts, they have the ability to transform others, be they male or female, into different kinds of monster, demons or beasts.
When her elemental powers reached its peak, Princess Bubblegum (Adventure Time) was able to turn everyone and everything around her into candy
, as well as make them more sweet in behavior. She was also shown to be able to convert even other Elementals.
When her elemental powers reached its peak, Patience St. Pim (Adventure Time) was able to turn everyone and everything around her into ice
, as well as make them more depressed in behavior.
When her elemental powers reached its peak, Slime Princess (Adventure Time) was able to turn everyone and everything around her into slime
, as well as make them more relaxed and festive in behavior.
When her elemental powers reached its peak, Flame Princess (Adventure Time) was able to turn everyone and everything around her into fire
, as well as make them more enraged and blood-hungry in behavior.
Pontiff's Left Eye (Dark Souls III) is a type of ring created by Pontiff Sulyvahn to be given to his Outrider Knights, transforming them into frenzied beasts...
...such as the unnamed Boreal Outrider Knight...
...Vordt of the Boreal Valley...
...and the Dancer of the Boreal Valley.
The Succubi (Monster Girl Encyclopedia) are hailed both by other mamono and the Demon Lord as the ultimate monster for their ability to procreate more and more monster girls, even monster boys who they take as their mates.
The energy emitted by Steppenwolf's (DC Extended Universe) Electro Axe can convert humans into Parademons.
The Muur Talisman (Star Wars Legends) is a sith artificial that allows its owner to transform any non-force sensitive beings into mindless Rakghouls and control them as their subservient slaves.
Celeste Morne (Star Wars Legends) using the Muur Tailsman to transform Darth Vader's Stormtroopers into Rakghouls.
The Machine Used for Literally Converting Humans To Plants, aka M.U.L.C.H., (Milo Murphy's Law: The Phineas and Ferb Effect) was created by Derek to transform humans into Pistachions.
The God Tree (Naruto) can convert any human ensnared in the Infinite Tsukuyomi into White Zetsu.