1An organism that lives in or on an organism of another species (its host) and benefits by deriving nutrients at the other's expense.

Parasites exist in huge variety, including animals, plants, and microorganisms. 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

'the parasite attaches itself to the mouths of fishes'

'an intestinal parasite of cattle'