Parasite

A parasite was an organism that acquired sustenance from a host organism, usually at the host's expense. They were regulated by their metabolisms, so an ecosystem was not overrun by them. (PROSE: Parasite)

Parasites could operate on any level, either biological or even universal. The Red leech fed off of a single host, (TV: The Crimson Horror) while entire races like the Nimons and the Stingrays used up whole civilisations, with the latter consuming the entire surface of a planet and reducing it to dust, (TV: The Horns of Nimon, Planet of the Dead) and the Elysium parasites laid eggs the size of planets into gas giants so their young could consume the world. (PROSE: Parasite)

Metaphorical parasites
The term "parasite" was sometimes used to describe persons who exploited others, such as when Mehendri Solon called the Time Lords "spineless parasites". (TV: The Brain of Morbius)

Other beings that were called parasites included the Animus by Vicki Pallister, (TV: The Web Planet) the Eternals by the Fifth Doctor, (TV: Enlightenment) and the Great Intelligence by the Eleventh Doctor (TV: The Snowmen) and Lethbridge-Stewart. (HOMEVID: Downtime)

Martha Jones compared Ernest Tiermann and his family to parasites living inside the body of a living thing, i.e. Dreamhome. (PROSE: Sick Building)

The First Doctor compared to a parasite, leeching off the advancements of the Citizens of Destination for his own ends. The Master agreed but noted that he should have chosen a better host. (AUDIO: The Destination Wars)