A parasite is an organism that lives in or on the living tissue of a host organism at the expense of it. The biological interaction between the host and the parasite is called parasitism.
Parasites are generally smaller than their hosts, absorbing nutrients from the host's body fluids, but this is far from a universal strategy.
See also: saprophyte, intestinal parasiteExamples
Many lifeforms are parasitic only during a part of their lifecycle. Many cuckoos, for example, are brood parasites: their young are parasitic on the host species, but adult cuckoos fend for themselves.
