The isolation of pathogenic strains of V. cholerae in drinking water is a major public health concern. However, some non-pathogenic serotypes of V. cholerae are part of the normal flora in some water systems. V. cholerae and other pathogenic vibrio spp. should be totally absent in drinking water supplies. The infectious dose of V. cholerae , though depends upon several factors such as gastric acidity, presence of alkaline substances in the food, extent of food intake, and of course individual tolerance. Study involving some volunteers reveals that exposure of a dose as low as `10^3` pathogenic V. cholerae01 bacteria have culminated in 60% actual attack in individuals, albeit in a milder form.