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Related to accidental parasite: endoparasite, facultative parasite, incidental parasite, accidental host


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 fevertyphoid fever
acute, generalized infection caused by Salmonella typhi. The main sources of infection are contaminated water or milk and, especially in urban communities, food handlers who are carriers.
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, tuberculosistuberculosis
(TB), contagious, wasting disease caused by any of several mycobacteria. The most common form of the disease is tuberculosis of the lungs (pulmonary consumption, or phthisis), but the intestines, bones and joints, the skin, and the genitourinary, lymphatic, and
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, and some types of pneumonia). Parasitic plants cause great losses among food crops and trees (see diseases of plantsdiseases of plants.
Most plant diseases are caused by fungi, bacteria, and viruses. Although the term disease is usually used only for the destruction of live plants, the action of dry rot and the rotting of harvested crops in storage or transport is similar to the rots
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). Parasites are more prevalent in the animal and protist kingdoms; most are invertebrates, chiefly worms, e.g., the flukefluke,
parasitic flatworm of the trematoda class, related to the tapeworm. Instead of the cilia, external sense organs, and epidermis of the free-living flatworms, adult flukes have sucking disks with which they cling to their hosts and an external cuticle that resists digestion
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, tapewormtapeworm,
name for the parasitic flatworms forming the class Cestoda. All tapeworms spend the adult phase of their lives as parasites in the gut of a vertebrate animal (called the primary host).
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, and trichina (see trichinosistrichinosis
or trichiniasis
, parasitic disease caused by the roundworm Trichinella spiralis. It follows the eating of raw or inadequately cooked meat, especially pork.
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); arthropods, e.g., the fleaflea,
common name for any of the small, wingless insects of the order Siphonaptera. The adults of both sexes eat only blood and are all external parasites of mammals and birds. Fleas have hard bodies flattened from side to side and piercing and sucking mouthparts.
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 and louselouse,
common name for members of either of two distinct orders of wingless, parasitic, disease-carrying insects. Lice of both groups are small and flattened with short legs adapted for clinging to the host.
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; and protozoans. Among the protozoanprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
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 parasites that cause human disease are Amoeba (or Entamoeba) histolytica, the cause of amebic dysenterydysentery
, inflammation of the intestine characterized by the frequent passage of feces, usually with blood and mucus. The two most common causes of dysentery are infection with a bacillus (see bacteria) of the Shigella group, and infestation by an ameba,
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 and liver abscess, and the several species of Plasmodium responsible for the three main types of malariamalaria,
infectious parasitic disease that can be either acute or chronic and is frequently recurrent. Malaria is common in Africa, Central and South America, the Mediterranean countries, Asia, and many of the Pacific islands.
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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 epiphyteepiphyte
or air plant,
any plant that does not normally root in the soil but grows upon another living plant while remaining independent of it except for support (thus differing from a parasite).
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, 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 saprophytessaprophyte
, any plant that depends on dead plant or animal tissue for a source of nutrition and metabolic energy, e.g., most fungi (molds) and a few flowering plants, such as Indian pipe and some orchids.
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 or saprobes. See also symbiosissymbiosis
, the habitual living together of organisms of different species. The term is usually restricted to a dependent relationship that is beneficial to both participants (also called mutualism) but may be extended to include parasitism, in which the parasite depends upon
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See R. Drisdelle, Parasites (2010).



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.



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.


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


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).