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Overview of Parasitic Infections

By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

  • Parasitic infections are more common in rural or developing areas than in developed areas.

  • In developed areas, these infections may occur in immigrants, in returning travelers, or in people with a weakened immune system.

  • Parasites usually enter the body through the mouth or skin.

  • Doctors diagnose the infection by taking samples of blood, stool, urine, phlegm, or other infected tissue and examining or sending them to a laboratory for analysis.

  • Travelers to areas where food, drink, and water may be contaminated are advised to cook it, boil it, peel it, or forget it.

  • Drugs that eliminate parasites are available for most infections.

A parasite is an organism that lives on or inside another organism (the host) and harms the host.

Parasitic infections are common in rural or developing areas of Africa, Asia, and Latin America and are less common in developed areas. A person who visits such an area can unknowingly acquire a parasitic infection, and a doctor may not readily diagnose the infection when the person returns home. In developed areas, parasitic infections may also affect immigrants and people with a weakened immune system (such as those who have AIDS or who take drugs that suppress the immune system—called immunosuppressants). The infections may occur in places with poor sanitation and unhygienic practices (as occurs in some mental institutions and day care centers).

Parasites usually 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 (called vectors because they convey or transmit organisms that cause disease). 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 containing the parasites. Rarely, parasites are spread through blood transfusions, in transplanted organs, through injections with a needle previously used by an infected person, or from a pregnant woman to her fetus.

Parasites that infect people include

  • Protozoa (such as amebas), which consist of only one cell

  • Worms (helminths, such as hookworms and tapeworms), which are larger and consist of many cells and have internal organs

Protozoa, which reproduce by cell division, can reproduce inside people. Helminths, in contrast, produce eggs or larvae that develop in the environment before they become capable of infecting people. Development in the environment may involve another animal (an intermediate host). A few helminths can develop in the human intestine and cause infection in that person (called autoinfection). Some protozoa (such as those that cause malaria) and some helminths (such as those that cause river blindness) have complex life cycles and are transmitted by insect vectors.


  • Laboratory analysis of samples of blood, stool, urine, or phlegm

Doctors suspect a parasitic infection in people who have typical symptoms and who live or have traveled to an area where sanitation is poor or where such an infection is known to occur.

Laboratory analysis of specimens, including special tests to identify proteins released by the parasite (antigen testing) or genetic material (DNA) from the parasite, may be needed. Samples of blood, stool, urine, or phlegm (sputum) may be taken. Doctors may test blood samples for antibodies to the parasite. Antibodies are proteins produced by the immune system to help defend the body against a particular attack, including that by parasites. The doctor may also take a sample of tissue that may contain the parasite. For example, a biopsy may be done to obtain a sample of lung or intestinal tissue. A sample of skin may be snipped. Several samples and repeated examinations may be necessary to find the parasite.

If parasites live in the intestinal tract, their eggs or cysts (a dormant and hardy form of the parasite) may be found in the person’s stool when a sample is examined under a microscope. Or parasites may be identified by testing the stool for proteins or other materials that parasites release. Antibiotics, laxatives, and antacids should not be used until after the stool sample has been collected. These drugs can reduce the number of parasites enough to make seeing the parasites in a stool sample difficult or impossible.


In areas of the world where food, drink, and water may be contaminated with parasites, wise advice for travelers is to

  • Avoid drinking tap water

  • "Cook it, boil it, peel it, or forget it"

Because some parasites survive freezing, ice cubes can sometimes transmit disease unless the cubes are made from purified water. Information about precautions needed in specific areas is available from the Centers for Disease Control and Prevention Travelers' Health page.


  • Antiparasitic drugs

For some parasitic infections, no treatment is needed. The infection may disappear on its own.

Some drugs (antiparasitic drugs) are designed particularly to eliminate parasites. Also, certain antibiotics and antifungal drugs (such as fluconazole) are effective against some parasitic infections.

No single drug is effective against all parasites. For some parasitic infections, no drug is effective.

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