A continuing concern is the conditions that result in newly emergent virulent strains of pathogens. Faruque et al. have provided molecular evidence that V. cholerae O139 strains are derived from O1 strains through genetic modification (29) . In addition, Chakraborty et al. in Kolkata have seen the presence and expression of virulence genes in several environmental strains of V. cholerae cul-tured from surface waters (30) . Recently, E. coli O157 has been isolated from the Ganges River in India for the first time (31) . Indications are that it is metabolically different from E. coli O157 isolated from other parts of the world, but the conditions that have led to these differences are as yet unclear. From the above studies, risk for transmission of virulence genes is likely to be high, but studies of conditions promoting transmission and approaches to modeling resultant disease risks are in their infancy. New epidemic strains could potentially occur through mutation of existing epidemic strains or through gene transfer. Environmental stressors such as chemical contaminants are thought to accelerate both mutation rates and gene transfer (32) . Thus, the degree of chemical pollution may need to be a component of disease models (in addition to other stressors).