Viroid is an evil video-virus that invaded Kevin Keene's body.
Viroid entered Kevin's body after he touched a mysterious rock from Kongoland. It piloted a crablike vehicle that it used to catch and imprison Kevin's life force, which it planned to absorb. It captured the shrunken N Team with the help of Cell and its germ minions, but was defeated when Princess Lana used Simon Belmont's mirror to reflect one of its shots back at it.
The appearance of Viroid's vehicle may be a reference to cancer, which derives its name from its crablike growth.