An infection of the intestinal tract that results from the parasite, Giardia lamblia. This parasite is found in contaminated water. It is becoming increasingly common among day care and preschool children. Giardiasis is spread through ingestion of contaminated food or water, or through contact with contaminated feces. Transmission through contaminated water is most common, and is frequent among people traveling in areas with poor sanitation or drinking directly from streams, especially in mountainous Western states.