Symbiont

A symbiont was a parasite-like creature which shared a mutually beneficial relationship with its host.

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)

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