Giardia lambia

Giardia lamblia is the most commonly diagnosed intestinal parasite in the United States. It has only two life-cycle stages: the binucleate trophozoite with four flagellae and the drug-resistant, four-nucleate cyst Ingestion, usually from contaminated drinking water, leads to infection. The trophozoites exist in the small intestine and divide by binary fission. Occasionally, cysts are formed that pass out in stools. Although some infections are asymptomatic, severe diarrhea can occur, which can be very serious in immune-suppressed patients.