parasitize v.

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 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]

word-forming element used to make verbs, Middle English -isen, from Old French -iser/-izer, from Late Latin -izare, from Greek -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached.

The variation of -ize and -ise began in Old French and Middle English, perhaps aided by a few words (such as surprise, see below) where the ending is French or Latin, not Greek. With the classical revival, English partially reverted to the correct Greek -z- spelling from late 16c. But the 1694 edition of the authoritative French Academy dictionary standardized the spellings as -s-, which influenced English.

In Britain, despite the opposition to it (at least formerly) of OED, Encyclopaedia Britannica, the Times of London, and Fowler, -ise remains dominant. Fowler thinks this is to avoid the difficulty of remembering the short list of common words not from Greek which must be spelled with an -s- (such as advertise, devise, surprise). American English has always favored -ize. The spelling variation involves about 200 English verbs.