Trophozoites are the active form of the parasite inside the body. Cysts represent the resting stage that enables the parasite to survive outside the body. Infection begins with the ingestion of the cysts. The acid in the stomach activates the cysts, which in turn release the trophozoites. The parasites attach to the lining of the small intestine, reproduce, and are swept down the intestine in the fecal stream.
Cysts form in the lower intestines and are then passed in the feces.
Epidemiology Giardia , one of the most common intestinal parasites worldwide, infects up to 20 percent of the world's population. The disease is most prevalent in developing countries, where infections are associated with poor sanitary conditions, poor water quality control, and overcrowding. In the United States, Giardia is a major cause of waterborne outbreaks of diarrhea. Giardiasis occurs primarily in mountainous areas where water supplies have become contaminated with feces from humans or possibly from animals, such as beavers. Because outbreaks have occurred from Washington state to New York state, campers and backpackers should avoid drinking any untreated water from mountain streams.