Parasite

A parasite was an organism that had a symbiotic relationship with a host organism that was harmed in the process. In some cases, the parasite could take control of the body, but this was uncommon.

On Argelius II a non-corporeal entity that fed on fear assumed the form of one of the planet's inhabitants. It was later discovered the same entity had taken the form of "Jack the Ripper" in Terran history. The entity traveled between planets taking over host humanoid bodies. The entity then used the host body to murder women to feed on their fear. The entity took complete control of the body, making it harmful and the host was no longer conscious of its surroundings. 

During a scan of Aldean DNA, Doctor Beverly Crusher was able to confirm that parasitic organisms were not responsible for the Aldeans' sterility. 

The neural parasites that invaded Starfleet Command were another type of parasite. Officers of Starfleet were taken over by neural parasites that attached themselves to the spinal cord and brain of the host. The host was left unable to control his own actions, and in the case of the Queen parasite, the host's body was left destroyed when the Queen exited. 

In 2377, B'Elanna Torres fainted while in engineering. Icheb scanned her with a medical tricorder and detected a lifeform inside her. He believed it to be a parasite, when in fact, she was pregnant with her first child. 

Parasites

 * Neural parasites
 * Xindi neural parasite
 * Neural parasite (23rd century)
 * Neural parasite (24th century)
 * Nitrium parasite
 * Silicon-based parasite
 * Tarchannen III parasite