. Cholera, the most severe of all diarrheal diseases, caused by Vibrio cholerae 01 (and most recently also by serogroup 0139) is one of the few pandemic infections of man. Its known history includes waves of severe, often fatal, disease covering the entire globe over decades of time, usually having originated from the cholera endemic area of the Ganges delta. In the heavily endemic regions of southeast Asia, cholera also exhibits a regular twice-yearly periodicity. Unfortunately, the mechanisms controlling the periodicity and pandemicity of cholera are not known. No studies to date have been able to explain this epidemiologic behavior of cholera. The primary objective of this proposal is to test the hypothesis that environmental factors involving surface waters are responsible for the observed epidemiology of cholera. We postulate that plankton living in surface waters are the reservoir for cholera vibrios, and that their growth and life cycle(s) control vibrio populations in surface water, and thus determine how vibrios are spread and when outbreaks of disease will occur. This information could 1) lead to an improved warning system for prediction of cholera outbreaks, both in endemic areas, and in areas into which a cholera pandemic is threatening, and possibly lead to new strategies for controlling the disease through environmental interventions directed toward growth of plankton. This joint project, involving also investigators from the International Center for Diarrheal Disease Research, Bangladesh, (ICDDR,B), the University of Maryland and Emory University, involves establishing four sentinel surveillance sites in bangladesh, three of which are known to exhibit cholera periodicity, and one that is usually free of cholera. A combined year-round effort will monitor 1) the clinical cases/infections with V.cholerae, 2) the occurrence and density of V. cholerae in the surface waters of these sites, including those attached to plankton, both as viable and as non-culturable forms, using sensitive sampling techniques (including PCR), 3) the growth of phytoplankton and zooplankton in these areas, using direct sampling and identification techniques as well as remote sensing by satellite, and 4) the genetic and phenotypic markers of isolates to determine whether V.cholerae isolated from surface waters are identical with those isolated from patients with cholera in the same vicinity. Based on these data, a model of cholera epidemiology will be developed which may be useful in predicting outbreaks of cholera, thereby allowing early mobilization of preventive and treatment measures. Solving this problem would be an enormous step toward our understanding of "emerging" infectious diseases on the planet.