Giardia Giardia Infections Factsheet Giardiasis is an infectious diarrheal disease. It is caused by a parasite that is most commonly transmitted through oral-fecal contact and by water contaminated by feces.
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.