Giardiasis (also known as beaver fever) 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.

Symptoms include loss of appetite, lethargy, fever, explosive diarrhea, loose or watery stool, stomach cramps, upset stomach, projectile vomiting (uncommon), bloating, and flatulence. Symptoms typically begin 1–2 weeks after infection and may wane and reappear cyclically. Symptoms are caused largely by the thick coating of Giardia organisms coating the inside of the small intestine and blocking nutrient absorption. Most people are asymptomatic; only about a third of infected people exhibit symptoms. Untreated, symptoms may last for six weeks or longer.