Symbiont

A symbiont, symbiote or symbiotic (TV:The Crimson Horror ^[+]Mark Gatiss, Doctor Who series 7 (BBC One, 2013).) was a parasite-like creature which shared a mutually beneficial relationship with its host.

The Iyteans were a symbiont race. (GAME: The Iytean Menace ^[+]J. Andrew Keith, The Doctor Who Role Playing Game (FASA, 1985).)

Leet was a symbiont which inhabited the body of Oscar Phillips. Without Leet, Oscar Phillips would have died in the destruction of the Regal Cinema, and without Oscar, Leet would have perished in the "cold wastes" of planet Earth. (PROSE: Bay of the Dead)

Red leech's were a symbiotic species. (TV: The Crimson Horror ^[+]Mark Gatiss, Doctor Who series 7 (BBC One, 2013).)

During the Kotturuh crisis, the Dalek Symbiont was created by the Dalek Time Squad. (PROSE: All Flesh is Grass)