Giardia parasites live in the intestines of people and animals. Before the microscopic parasites are passed in stool, they become encased within hard shells called cysts, which allows them to survive outside the intestines for months. Once inside a host, the cysts dissolve and the parasites are released.

Infection occurs when you accidentally ingest the parasites. This can occur by swallowing contaminated water, by eating contaminated food or through person-to-person contact.

Swallowing contaminated water

The most common way to become infected with giardia is after swallowing contaminated water.

Giardia parasites are found in lakes, ponds, rivers and streams worldwide, as well as in municipal water supplies, wells, cisterns, swimming pools, water parks and spas. Ground and surface water can become contaminated from agricultural runoff, wastewater discharge or animal feces. Children in diapers and people with diarrhea may accidentally contaminate pools and spas.

Eating contaminated food

Giardia parasites can be transmitted through food — either because food handlers with giardiasis don't wash their hands thoroughly or because raw produce is irrigated or washed with contaminated water. Because cooking food kills giardia, food is a less common source of infection than water is, especially in industrialized countries.

Person-to-person contact

You can contract giardiasis if your hands become contaminated with fecal matter — parents changing a child's diapers are especially at risk. So are child care workers and children in child care centers, where outbreaks are increasingly common. The giardia parasite can also spread through anal sex.

Nov. 14, 2012