Introduction
A parasite is an organism that lives on or inside another organism (the host) and causes harm to the host.
Parasitic infections are common in rural parts of Africa, Asia, and Latin America and less prevalent in industrialized countries. A person who visits a developing country can unknowingly acquire a parasitic infection; a doctor may not readily diagnose the infection when the person returns home.
Parasites generally enter the body through the mouth or skin. Parasites that enter through the mouth are swallowed and can remain in the intestine or burrow through the intestinal wall and invade other organs. Parasites that enter through the skin bore directly through the skin or are introduced through the bites of infected insects (the vector). Some parasites enter through the soles of the feet when a person walks barefoot or through the skin when a person swims or bathes in water where the parasites are present.
A doctor who suspects that a person may have a parasitic infection may take samples of blood, stool, or urine for laboratory analysis. The doctor may also take a sample of tissue that may contain the parasite. Repeated sample collections and examinations may be necessary to find the parasite.
Some parasites, particularly those that are single-celled, reproduce inside the host. Other parasites have complex life cycles, producing eggs or larvae that spend time in the environment or in an insect vector before becoming infective. If egg-laying parasites live in the digestive tract, their eggs may be found in the person's stool when a sample is examined under a microscope. Antibiotics, laxatives, and antacids can reduce the number of parasites enough to make their detection in a stool sample more difficult.
Food, drink, and water are often contaminated with parasites in regions of the world with poor sanitation and unhygienic practices. Thus, wise advice for travelers is to "cook it, boil it, peel it, or forget it" when eating in developing areas. Because some organisms survive freezing, ice cubes sometimes transmit disease unless made from purified water.
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