What You Should Know About. Giardiasis is an intestinal illness caused by infection with a parasite called Giardia lamblia. The infective form is called a cyst, a stage in the parasite's life cycle which allows it to survive in the environment, primarily fecally contaminated water.
Infection is acquired by swallowing cysts.
Upon reaching the intestine, the cyst transforms into the second stage of the parasite's life cycle, which is called a trophozoite. The trophozoite stage of Giardia grows and multiplies in the intestine, and is responsible for the symptoms of the illness. Anyone can get giardiasis, but it is more frequently found in persons living in institutional settings for the developmentally disabled, individuals in child care settings, and travelers to areas with poor sanitation. Anyone who drinks untreated surface water, such as water taken directly from ponds, lakes and streams, is at increased risk of acquiring giardiasis. Transmission of giardiasis has also been documented in male homosexuals.