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parasite, plant or animal that at some stage of its existence obtains its nourishment from another living organism called the host. Parasites may or may not harm the host, but they never benefit it. They include members of many plant and animal groups, and nearly all living things are at some time hosts to parasitic forms. Many bacteria are parasitic on external and internal body surfaces; some of these invade the inner tissues and cause disease (e.g., typhoid fever, tuberculosis, and some types of pneumonia). Parasitic plants cause great losses among food crops and trees (see diseases of plants). Parasites are more prevalent in the animal and protist kingdoms; most are invertebrates, chiefly worms, e.g., the fluke, tapeworm, and trichina (see trichinosis); arthropods, e.g., the flea and louse; and protozoans. Among the protozoan parasites that cause human disease are Amoeba (or Entamoeba) histolytica, the cause of amebic dysentery and liver abscess, and the several species of Plasmodium responsible for the three main types of malaria.

Most parasites are obligate; i.e., they are unable to survive apart from their hosts. Often this is because in the course of evolution they have lost various of the organs necessary to live as independent units. Many parasites also have extremely specialized reproductive systems and complex life cycles, involving more than one host. Some higher plants and animals are parasitic, e.g., the dodders (vines of the morning glory family) and the cuckoo and the cowbird, which lay their eggs in the nests of other birds.

An epiphyte, or air plant, although it lives in association with another plant, is not a parasite. Organisms that obtain their nourishment from dead organic matter, e.g., mushrooms, are called saprophytes or saprobes. See also symbiosis.


See R. Drisdelle, Parasites (2010).

The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. Licensed from Columbia University Press. All rights reserved.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



an organism that feeds on and usually harms another organism, which is called the host. A distinction is made between zooparasites, including protozoans, helminths, arachnids, and insects, and phytoparasites, including bacteria, fungi, and some higher plants. Viruses are also classified as parasites.

Parasites are found in all taxonomic groups except echinoderms, brachiopods, most chordates, mosses, ferns, and gymnosperms. Bacteria, actinomycetes, plants, and animals of all taxonomic groups may serve as hosts. Parasites weaken and exhaust the host and often kill it. Some parasites require a succession of two or three hosts on which to complete their life cycle.

Parasites arose in the course of evolution from free-living forms. In adapting to their new living conditions, their internal organization was simplified, and they acquired special organs for attachment, in addition to well-developed sex organs. Anaerobic respiration enables parasites to exist in environments that lack oxygen. Many parasites cause diseases of man, animals, and plants.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


An organism that lives in or on another organism of different species from which it derives nutrients and shelter.
Current in a circuit, due to some unintentional cause, such as inequalities of temperature or of composition; particularly troublesome in electrical measurements.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


an animal or plant that lives in or on another (the host) from which it obtains nourishment. The host does not benefit from the association and is often harmed by it
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


Unsolicited software that is installed in a computer without users realizing it. There are many different types. Parasites can report Web browsing habits to a marketing company over the Internet (see spyware) or change browser settings to point to a specific site. They can redirect search engine results to a site that sells a related product, and they can cause premium services to be dialed up.

Read the License Agreement
Parasites are often installed with freeware, and the license agreement may actually say so, but hardly anyone reads it. Sometimes, users can opt out of installing the parasite and install only what they wanted in the first place. Be sure security settings are set to medium at least, and never click "Yes" to any dialog that asks "do you want to run" or "execute" something unless you know what that something is. ActiveX controls on the Web cannot only install parasites but viruses as well.

Parasites often do not include an uninstall function and may not be easily removed, although anti-parasite programs can detect and remove them (see spyware blocker).
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