"Each gram of human feces contains approximately 12 billion (12*109) bacteria, among them may include pathogenic bacteria, such as Salmonella, associated with gastroenteritis. In addition, feces may contain pathogenic viruses, protozoa and parasites. If ingested, these organisms would cause disease. When testing drinking water for contamination, the variety and often low concentrations of pathogens makes them difficult to test for individually. Health authorities therefore use the presence of other more abundant and more easily detected fecal bacteria as indicators of the presence of fecal contamination. Indicator bacteria are not themselves dangerous to the health but are used to indicate the presence of a health risk. The most popularized known indicator bacteria are fecal coliforms, which are found in the intestinal tracts of warm blooded animals. Another less commonly used group of indicator organisms are hydrogen sulfide producing bacteria, which are also found in humans as well as the intestinal tracts of birds and reptiles - known carriers of Salmonella. WHO Guidelines for Drinking Water Quality state that as an indicator organism Escherichia coliform provides conclusive evidence of recent fecal pollution and should not be present in water meant for human consumption. It is generally assumed that higher the number of coliform organisms found in a 100L sample, the higher the risk for waterborne disease." Source and further information: http://en.wikipedia.org/wiki/Indicator_bacteria Further information: http://en.wikipedia.org/wiki/Coliform_bacteria