Giardiasis or beaver fever in humans is a diarrheal infection of the small intestine by a single-celled organism Giardia lamblia. Giardiasis occurs worldwide with a prevalence of 20–30% in developing countries. In the U.S., 20,000 cases are reported to the CDC annually, but the true annual incidence is estimated at 2 million people. Giardia has a wide range of mammalian hosts besides humans, thus making it very difficult to eradicate. For people with compromised immune systems, such as elderly or AIDS patients, giardiasis can be deadly.