Giardiasis is caused by an intestinal infection of flagellate protozoa belonging to the Giardia spp. The parasite is distributed worldwide. The lifecycle of the organism includes trophozoites in the vegetative state and cysts in the transmission stage. The trophozoites live in the upper part of the small intestine, especially the duodenum, and are carried to the ileum where they secrete a heartening substance and become encysted. Cysts leave the infected host in the feces. They can survive more than two months in cool water, and about one month at room temperature, and are sensitive to desiccation, sunlight, and freezing. They are readily killed by quaternary ammonium compounds but not by normal concentration of chlorine in drinking water.

Endemic throughout the world, the prevalence of giardiasis ranges from 2-4 % in industrialized countries, to 15% or higher in children of developing countries. Regardless of location, both infection and disease are more commonly seen in children. Epidemics of giardiasis also can occur; the latest outbreaks in the United States were in 1993-1994 and 1995-1996 from drinking infected water or exposure to infected recreational waters. Contaminated drinking water supplies also caused Giardia spp. outbreaks in the United States. Children and travelers to other countries can be infected from contaminated drinking or other waters, whereas patients with AIDS are less likely to be infected, perhaps because HIV virus interferes with the parasite's ability to multiply in the intestine.

The primary source or reservoir of infection in humans is other infected humans, who shed parasite cysts in their feces and often contaminate water supplies. The infection is usually mild and self-limiting. While as little as 10 cysts can be infective, infected individuals can excrete up to 900 million cysts daily in the feces! Hand-to-hand and hand-to-mouth transmission occurs from infected to susceptible people (usually children) in institutions that care for children and adults. Fortunately, the organism is susceptible to desiccation and sunlight and so does not survive on contaminated food or other fomites for very long.

Some animals may also serve as a reservoir for human infection. Known outbreaks have occurred from infected beavers contaminating mountain water, and from non-human primates infecting zoo personnel. Specific-pathogen- free puppies have been infected from Giardia cysts isolated from beavers.

A wide variety of domestic and wild mammal species are known to be hosts of Giardia spp. The infection is less common in adult animals, and prevalence surveys throughout the world have found the parasite in 20-35% of young dogs, 10-15% of young cats, 5-90% of calves, 6-80% of lambs, 17-32% of foals, and 7-44% of young pigs. In the United States, beavers and coyotes have been shown to harbor the organism, and an outbreak among non-human primates and zoo personnel also has been documented. Rats and other rodents can harbor a high rate of infection in both domestic and wild stock.