Giardia outbreaks can occur in communities in both developed and developing countries where water supplies become contaminated with raw sewage. It can be contracted by drinking water from lakes or streams where water-dwelling animals such as beavers and muskrats, or where domestic animals such as sheep, have caused contamination. It is also spread by direct person-to-person contact, which has caused outbreaks in day-care centers.

Travelers are at risk for giardiasis throughout the world. Campers and hikers are at risk if they drink untreated water from streams and lakes. Other risk factors include unprotected anal sex, exposure to a family member with giardiasis, and institutional (day-care or nursing home) exposure. The incidence has been on the increase in the last few years.