Giariasis — popularly known as beaver fever or backpacker's diarrhea — is a disease caused by the flagellate protozoan Giardia lamblia (also sometimes called Giardia intestinalis and Giardia duodenalis). The giardia organism inhabits the digestive tract of a wide variety of domestic and wild animal species, including humans. It is a common cause of gastroenteritis in humans, infecting approximately 200 million people worldwide.