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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Giardiasis is an intestinal illness caused by infection with the parasite Giardia lamblia, which lives in contaminated water. Although the illness most often occurs in developing countries, giardiasis is also a common cause of waterborne illness in the United States. A person can remain infected with Giardia until the infection is diagnosed and treated. In developing areas of the world, it is common for more than 20% of a country's population to have ongoing Giardia infection. In the United States, only 1 or 2 out of every 10,000 people have Giardia in a typical year, but the infection is found in about 1 out of 3 people who have prolonged diarrhea symptoms if they have recently traveled to a developing country.

As one part of their life cycle, G. lamblia parasites change into cysts. Contagious cysts are found in the feces of infected people or animals. You can become infected with G. lamblia by:

  • Drinking water that has been contaminated with Giardia cysts (usually because the water has come into contact with sewage)

  • Eating uncooked fruits or vegetables that have been washed in contaminated water

  • Eating uncooked fruits or vegetables from a garden where contaminated fertilizer has been used

  • Touching feces, diapers, or objects soiled with feces, then failing to adequately wash your hands

  • Having direct contact with an infected person or animal, then failing to adequately wash your hands

G. lamblia can survive in cold, chlorinated water for up to two months, and outbreaks have occurred in municipal water supplies.

People at greatest risk of giardiasis include:

  • Children in day care centers and their families

  • Day care workers

  • Travelers to developing countries

  • Campers who drink unprocessed water

  • Homosexual men (because of anal sex)

Children are three times more likely to develop giardiasis than adults. It is possible that the human body develops some immunity to the parasite over time.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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