Travelers are cautioned against drinking any untreated water. It causes infection up to 20 percent of the world's population, and is most prevalent in developing countries, where infections are associated with poor sanitary conditions, poor water quality control, and overcrowding. Giardia lamblia is a major cause of waterborne outbreaks of diarrhea in the United States. It occurs primarily in areas where water supplies have become contaminated with feces from humans or, possibly, from animals. Outbreaks have occurred throughout the country.
Infection begins when the cysts are consumed through food or water.
Stomach acid activates the cysts and the trophozoites are released. They become attached to the lining of the small intestine and reproduce. Cysts form in the lower intestines and are then passed in the feces. The parasite may be directly transmitted from person to person by contact with infected feces, or indirectly, through consuming food and water contaminated with cyst-containing feces. The following are the most common symptoms of giardiasis.