<i>Giardia</i> lamblia is a microscopic parasite that causes Giardiasis which is one of the most common causes of waterborne gastrointestinal disease. It is found naturally in streams and lakes but it can also contaminate public water supplies, swimming pools, whirlpool spas, and wells. In children under the age of five giardiasis can lead to complications such as malnutrition however it generally only causes mild symptoms in adults. The best single diagnostic test for giardiasis is the antigen detection test using stool samples. Molecular testing is also available and is commonly used in vet diagnostics.