Organisms that live on or in a host organism and derive nutrients at the host's expense. The term can also refer metaphorically to people who depend on others without giving anything in return.
From Greek 'parasitos,' literally meaning 'beside food,' originally referring to a person who ate at another's table. The biological sense developed in the 18th century as scientific understanding of symbiotic relationships advanced.
Some parasites have evolved such intricate relationships with their hosts that they can actually control behavior - the toxoplasma parasite, for example, makes mice less afraid of cats, increasing the parasite's chances of completing its life cycle. Paradoxically, nearly half of all known animal species are parasites, making them one of the most successful life strategies on Earth.
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