A disease caused by the protozoan parasite Giardia lamblia, characterized by chronic diarrhea that usually lasts 1 or more weeks. The diarrhea may be accompanied by one or more of the following: abdominal cramps, bloating, flatulence, fatigue, or weight loss. The stools are malodorous and have a pale greasy appearance. Infection without symptoms is also common. As with most other protozoa inhabiting the intestinal tract, the life cycle of Giardia involves two stages: trophozoite and cyst. Trophozoites stay in the upper small-intestinal tract, where they actively feed and reproduce. When the trophozoites pass down the bowel, they change into the inactive cyst stage by rounding up and developing a thick exterior wall, which protects the parasite after it is passed in the feces. People become infected either directly by hand-to-mouth transfer of cysts from feces of an infected individual or indirectly by drinking feces-contaminated water. After the cyst is swallowed, the trophozoite is liberated through the action of digestive enzymes and stomach acids, and becomes established in the small intestine.