1530s, "a hanger-on, a toady, person who lives on others," from French parasite (16c.) or directly from Latin parasitus "toady, sponger," and directly from Greek parasitos "one who lives at another's expense, person who eats at the table of another," especially one who frequents the tables of the rich and earns his welcome by flattery, from noun use of an adjective meaning "feeding beside," from para- "beside" (see para- (1)) + sitos "grain, bread, food," a word of unknown origin.
Scientific meaning "animal or plant that lives on or in and at the expense of another" is first recorded 1640s (implied in parasitical).
There is scarcely any animal that may not or does not serve as the host of parasites, and some parasites are themselves the hosts of other parasites. ... Parasites form no technical group of animals, since representatives of almost any class or order, from protozoans to vertebrates, may be parasitic. Most of the leading divisions of animals, however, include some members, whether genera, families, orders, or even classes, whose habit is extensively or exclusively parasitic. [Century Dictionary]