Giardiasis (GEE-are-DYE-uh-sis) is a diarrheal illness caused by a one-celled, microscopic parasite, Giardia intestinalis (also known as Giardia lamblia). Giardia infection can cause a variety of intestinal symptoms, which include Diarrhea Gas or flatulence Greasy stools that tend to float Stomach cramps Upset stomach or nausea.