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