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1. a plant or animal that lives upon or within another living organism at whose expense it obtains some advantage; see also symbiosis. Parasites include multicelled and single-celled animals, fungi, and bacteria, and some authorities also include viruses.Those that feed upon human hosts can cause diseases ranging from the mildly annoying to the severe or even fatal. (See accompanying table.)
2. parasitic fetus.adj., adj parasit´ic.
accidental parasite one that parasitizes an organism other than the usual host.
facultative parasite one that may be parasitic upon another organism but can exist independently.
incidental parasite accidental parasite.
malarial parasite Plasmodium.
obligate parasite (obligatory parasite) one that is entirely dependent upon a host for its survival.
periodic parasite one that parasitizes a host for short periods.
temporary parasite one that lives free of its host during part of its life cycle.
an organism that may either lead an independent existence or live as a parasite, in contrast to obligate parasite.
fac·ul·ta·tive par·a·site(fak'ŭl-tā'tiv par'ă-sīt)
An organism that may either lead an independent existence or live as a parasite, in contrast to an obligate parasite.