Different types of parasites usually evolve in differing ecosystems where resources or safety are being sought after by multiple organisms. Parasites can feed off other beings, or use them as transportation or the advancement of their life processes. A similar biological arrangement is symbiosis, but symbiosis is defined as two organisms that can mutually benefit from such an arrangement.
Various types of neural parasites exist, sometimes capable of feeding on or taking control of the nervous systems of sentient beings. (TOS episode & Star Trek 2 novelization: Operation -- Annihilate!; TNG episode: "Conspiracy")
- Ardanan parasite
- neural parasite