Parasytes are a race of extraterrestrial beings that have a desire to take over a human host. They came from small eggs which floated down to Earth from unknown origins.
They start out as a small worm that enters the body through some kind of orifice (i/e: Ear, nose, mouth). If the host resists, the parasyte has a sharp spine which allows it to burrow directly under a human's skin. Then, they travel to the head and take it over, controlling the entire body with the central nervous system. Sometimes, they will accidentally take over a different part of the host, where it forms a symbiosis with the host, while both beings retain separate intellects. These parasytes are usually regarded as either pathetic failures, or dangerous threats.