MUCH publicity has been given to the epidemic incidence of giardiasis in travelers returned from the Soviet Union, and a number of water-borne outbreaks have occurred in the United States. Nevertheless, giardiasis remains an enigmatic infection.EpidemiologyTap water has been implicated as the main source of infection in Leningrad, but infections from other sources and in other parts of the Soviet Union certainly occur. However, giardiasis is not limited to the Soviet Union, and I have recognized infections contracted in 97 different countries. Areas of increased risk of infection for both longer-term residents and travelers in a series of . . .