Parasite

An organism that lives in or on and takes its nourishment from another organism. A parasite cannot live independently. Parasitic diseases include infections by protozoa, helminths, and arthropods: {{}}Protozoa — Malaria is caused by plasmodium, a protozoa, a single-cell organism that can only divide within its host organism. Helminths — Schistosomiasis, another set of very important parasitic diseases, is caused by a helminth (a worm). Arthropods — The arthropods include insects and arachnids (spiders, etc.), a number of which can act as vectors (carriers) of parasitic diseases. The term "parasite" came from the Greek "parasitos" (para-, along side of + sitos, food) meaning "eating at the side of, as at the same table." The sense of the term later changed to that of a poor friend or relative who lived at the expense of another. Not until the 18th century did "parasite" come into English as a biologic term. The study of parasites is parasitology.

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1. An organism that lives on or in another and draws its nourishment therefrom. 2. In the case of a fetal inclusion or conjoined twins, the usually incomplete twin that derives its support from the more nearly normal autosite. [G. parasitos, a guest, fr. para, beside, + sitos, food]

- accidental p. SYN: incidental p..

- autistic p. a p. descended from the tissues of the host. SYN: autochthonous p..

- autochthonous p. SYN: autistic p..

- commensal p. commensal (2).

- euroxenous p. a p. with a broad or nonspecific host range.

- facultative p. an organism that may either lead an independent existence or live as a p., in contrast to obligate p..

- heterogenetic p. a p. whose life cycle involves an alternation of generations.

- heteroxenous p. a p. that has more than one obligatory host in its life cycle.

- incidental p. a p. that normally lives on a host other than its normal host. SYN: accidental p..

- inquiline p. inquiline.

- malignant tertian malarial p. SYN: Plasmodium falciparum.

- obligate p. a p. that cannot lead an independent nonparasitic existence, in contrast to facultative p..

- quartan p. SYN: Plasmodium malariae.

- specific p. a p. that habitually lives in its present host and is particularly adapted for the host species.

- spurious p. organisms that parasitize other hosts that pass through the human intestine and are detected in the stool after ingestion ( e.g., Capillaria sp. eggs in animal liver).

- stenoxous p. a p. with a narrow or specific host range.

- temporary p. an organism accidentally ingested that survives briefly in the intestine.

- tertian p. SYN: Plasmodium vivax.

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par·a·site 'par-ə-.sīt n an organism living in, with, or on another organism in parasitism

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n.

any living thing that lives in (see endoparasite) or on (see ectoparasite) another living organism (see host). The parasite, which may spend all or only part of its existence with the host, obtains food and/or shelter from the host and contributes nothing to its welfare. Some parasites cause irritation and interfere with bodily functions; others destroy host tissues and release toxins into the body, thus injuring health and causing disease. Human parasites include fungi, bacteria, viruses, protozoa, and worms. See also commensal, symbiosis.

• parasitic adj.

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par·a·site (parґə-sīt) [Gr. parasitos] 1. a plant or animal that lives upon or within another living organism at whose expense it obtains some advantage. See symbiosis. 2. the smaller, less complete component of asymmetrical conjoined twins, which is attached to and dependent on the autosite.