parasite

noun

• 1an organism which lives in or on another organism (its host) and benefits by deriving nutrients at the other’s expense: the parasite attaches itself to the mouths of fishes an intestinal parasite of cattle

Parasites exist in huge variety and include animals, plants, and micro-organisms. They may live as ectoparasites on the surface of the host (e.g. arthropods such as ticks, mites, lice, fleas, and many insects infesting plants) or as endoparasites in the gut or tissues (e.g. many kinds of worm), and cause varying degrees of damage or disease to the host

• 2 derogatory a person who habitually relies on or exploits others and gives nothing in return: the capitalist is really a parasite on the workers