Despite considerable research, the routes of transmission and the basis for the seasonal and pandemic cycles of cholera have not been explained. Those areas of the world where clinical cholera cases appear without apparent importation on a regular seasonal basis are endemic foci for cholera. While cholera transmission can occur in many parts of the world, not all such places permit the inter-epidemic maintenance of Vibrio cholerae. Two types of cholera endemic focus exist. Primary endemic foci are present independent of the occurrence of a pandemic, and are found mainly in Bangladesh and India. Secondary endemic foci are only present during a pandemic and are currently found in parts of Africa, the USA, Australia and Europe (Miller, 1984). Common factors between the different foci may give clues as to how V. cholerae is maintained in these areas during the non-epidemic part of the year. Low-lying riverine areas are strongly associated with epidemic areas and frequently outbreaks are initiated in fishing communities. The mechanisms for the maintenance of endemicity may also be linked with seasonal variation in the distribution of cholera cases.