Giardia lamblia, a flagellate with cosmo-politan geographic distribution, is found in duodenal contents and bile. In the duodenum it can be demonstrated in the mucosal crypts where it attaches itself to the mucosal cells, causing gastroenteritis. When swept into the fecal stream, the trophozoite encysts. Consequently, most fecal specimens contain the encysted parasite rather than the flagellated trophozoite form, which is usually found only in severe diarrhoea. In the cyst (resistant) form, they spread the disease from host to host by fecal/oral routes, either directly (as between children in day-care centers or between sexual partners) or by food and water. Waterborne epidemics involve mountain streams, well water, and even some chlorinated community water systems.