This article includes information on two human parasites, one protozoan flagellate, Giardia lamblia, and one ciliate, Balantidum coli. Both are transmitted through ingestion of food and water contaminated with fecal material. G. lamblia may be the most common intestinal protozoan found in humans throughout the world and causes a wide range of symptoms, all of which can be confused with other infectious and noninfectious causes. Although B. coli tends to be more restricted and associated with pigs as potential reservoir hosts, this organism can also cause mild to severe symptoms and can be found throughout the world.