Parasite
A parasite, parasitical being, or a parasitic organism, was a type of organism  that has a symbiotic relationship with a host organism that is harmed in the process. In some cases, the parasite can take control of the body, but this is uncommon.

 considered children parasites, stating that she had to find someone else to feed hers or she never would have gotten anything done. ()

After Nomad was initially beamed aboard the , Captain James T. Kirk, believing it was some sort of small spacecraft, informed the device that "if you care to leave your ship, we'll provide the necessary life support systems." Nomad found the statement to be a "non sequitur," further explaining that "I contain no parasitical beings. I am Nomad." ()

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 parasitic beings 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 quickly believed it to be a parasite. Seven of Nine then did her own scan and found she was actually pregnant with her first child. ()

Parasites
 Comtaxan parasite
 Neural parasites
 Flying parasite
 Parasitic being
 Xindi neural parasite
 Nitrium parasite
 Silicon-based parasite
 Tarchannen III parasite
 Ceti eel
 Centaurian slug

External link
 
Lifeform classifications