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Giardia lamblia is a protozoan widely found in nature. Infection occurs through ingestion of food or water contaminated with the feces of infected mammals especially beavers, puppies, and cats, and sometimes humans. Fecal-oral transmission can occur in child-care centers, households, and during sexual contact. Child-care center outbreaks have been associated with toddler wading pools where diapered children share the same water.

Like other enteric infections, rates of giardiasis increase during warmer months, probably because of more frequent exposure to contaminated water through swimming or camping. Travelers to developing countries are at increased risk of giardiasis as well.