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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).
References in periodicals archive ?
Prevalence of external parasites in the southeastern desert of Egypt.
There are products that are targeted to treat intestinal worms only, and combination products that also control external parasites and/or prevent heartworm.
Brushing your cat is a special bonding time for both pet and person and also a good opportunity to examine carefully for any external parasites, scratches or bite wounds.
The frequency of internal parasitism was found highest (32.2%), followed in order by wounds (26.9%), bacterial infections (20.4%), lameness (12.9%), miscellaneous (12.7%), gastrointestinal disorders (7.6%), bronchitis/cough (7.3%), allergic dermatitis (7.1%), external parasites (6.2%), colic (5.3%), eye problems (3.1%), hematuria (1.8%) and quidding (1.1%).
The surviving shiners were in poor health: "They had been trapped in pools and warm water for a long time, and a lot of them showed evidence of external parasites," Wilde said.
Fleas, ticks and other external parasites are another concern.
Fish were injected monthly a 10% enrofloxacin solution (0.05 mL [kg.sup.-1]) to prevent infections, whereas 3-5 min fresh water baths were used to eliminate external parasites. Recently catch (CAT), parasitological analysis (PAT) and RAS fish were sacrificed to check internal parasites.
We occasionally see dogs that have been intentionally burned, starved or beaten, but more often than not the pets that are presented to us are malnourished, riddled with internal and external parasites and suffering from the consequences of existing in a dirty environment where health and hygiene are just not priorities.
The industry made a significant change for the better in parasite control in 1981 when Merck Animal Health (MSD Ag-Vet) introduced the new active ingredient ivermectin in various forms for the control of internal and external parasites. This class of products, known as avermectins, were revolutionary and provided improved control of a large number of parasites at much lower doses of active material along with a significantly improved safety profile to humans, animals and the environment.
Inspector General Forests at the Ministry of Climate Change, Syed Mahmood Nasir, besides Newcastle viral disease there are some other causes too that have led to deaths of the peacocks, such as: bacterial, protozoan diseases, international and external parasites as well as nutritional deficiency.
External Parasites of Small Ruminants: A Practical Guide to their Prevention and Control offers a guide to preventing and controlling ectoparasites that contribute to diseases in sheep and goats, and is a pick for goat and sheep herders, farmers, and veterinary collections alike.
External parasites of small ruminants; a practical guide to their prevention and control.