An organism that
lives on or in a host organism of a different species and that
gets its food from or at the expense of its host, and which is
typically detrimental to the host.
A parasite is an organism that lives intimately
associated in or on the living tissue of a host organism. A
parasite obtains nourishment and shelter from the host without
benefiting or killing it usually does not eat a large
proportion of its host tissue. A parasite does not help its host
in any way.
A complete parasite gets all of its nutrients from the host
organism, whilst a semi-parasite gets only some of its food from
The biological interaction between the host and the parasite is
called parasitism. Parasitism is almost a form of predation, it
is a type of symbiosis, by one definition, although another
definition of symbiosis excludes parasitism, since it requires
that the host benefit from the interaction as well as the
parasite. (Of course animals and plants may be parasitized or
Some parasitic insects are considered beneficial because
parasitize pest species.
Among plant parasites: Mealy bug, aphids, nematodes, etc.