The Giardia lamblia is a microorganism frequently found in rivers and lakes, which, if not treated properly, may cause diarrhea, fatigue, and cramps after ingestion.

People with severely weakened immune systems are likely to have more severe and more persistent symptoms than healthy individuals.

Other /More definition:
A Giardia lamblia is a protozoan frequently found in rivers and lakes, which can survive in water for 1 to 3 months, associated with the disease giardiasis. Ingestion of this protozoan in contaminated drinking water, exposure from person-to-person contact, and other exposure routes may cause giardiasis. The symptoms of this gastrointestinal disease may persist for weeks or months and include diarrhea, fatigue, and cramps.