Parasite

A parasite was a species which acquired sustenance from a host organism, usually at the host's expense. The Doctor encountered several parasitic life forms in his travels.

Parasitic species
category=Parasitic species columns=3 shownamespace=false

Metaphorical parasites
The term "parasite" was sometimes used to describe persons who exploited others. Vicki called the Animus a parasite. (TV: The Web Planet) The Eleventh Doctor called the Great Intelligence a parasite. (TV: The Snowmen)