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
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Metaphorical parasites
The term "parasite" was sometimes used to describe persons who exploited others. Vicki called the Animus a parasite. (TV: The Web Planet) Mehendri Solon called the Time Lords "spineless parasites". (TV: The Brain of Morbius) The Fifth Doctor called the Eternals parasites. (TV: Enlightenment) The Eleventh Doctor called the Great Intelligence a parasite. (TV: The Snowmen)