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)

At the heart of the Keller Machine was a psychic parasite that fed on strong emotions. (TV: The Mind of Evil) It and its ilk arrived on Earth in a starship crash in China 150 years before 1997, coming to be held in monasteries across China. In the Unbound Universe, following the Master defecting to China, he used the parasites to make more Keller Machines only for the parasites to become sated by 1997 with the Chinese planning to destroy them via a nuclear bomb, save one that was held in Hong Kong. Though the Master planned to use the last parasite to rule the world, the Doctor stopped his plan and deposited the parasite with its fellows to be destroyed. (AUDIO: Sympathy for the Devil)

Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS likened the Abzorbaloffs' absorption ability to a bacterium or a tapeworm, all essentially parasitic in nature. (PROSE: The Monster Vault)

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)