Parasites

Parasites are life-forms which require the assistance of other life-forms to reproduce, feed or survive, whether the host is willing or not, due to the idea of another living creature residing within oneself being a primal fear of many (as well as a deep instinctual fear of parasites and the devastating effect they can have upon population) these are a common antagonist in many forms of fiction, especially in the genre known as body horror (which specializes in the destruction or alteration of the body).