http://en.wikipedia.org/wiki/Giardia_lamblia
Giardia lamblia (formerly also Lamblia intestinalis and also known as Giardia duodenalis) is a flagellated protozoan parasite that infects the gastrointestinal tract of humans. It is also one of the most common parasites infecting cats. Infection causes giardiasis, a type of gastroenteritis that manifests itself with severe diarrhea and abdominal cramps. Other symptoms can include bloating, flatulence, fatigue, nausea, vomiting and weight loss. In some patients, vomiting or nausea is the major symptom. The symptoms usually manifest themselves about seven to ten days after the organism is ingested. Giardia is a major cause of intestinal disease worldwide and the most frequent non-bacterial cause of diarrhea in North America. Nonetheless, the basic biology of this parasite is poorly understood.
Infection from giardia can occur from consuming contaminated food or water. It can also be transferred from animal or human feces. Not every person displays symptoms of infection, but they can still serve as a carrier of the disease. Giardia infection is a concern for people camping in the wilderness or swimming in contaminated streams or lakes, especially the artificial lakes formed by beaver dams (hence the popular name for giardiasis, "Beaver Fever"). Filter use or boiling is recommended for purifying drinking water in wilderness conditions.