This is even true of water which has undergone treatment for drinking water purposes. There is no clear way to associate risk of disease with the bacteriological quality of ground water and measured by the presence of fecal indicator bacteria. First, there is no direct association between the presence of fecal indicator bacteria and the presence of specific pathogens. Second, individuals are not equally susceptible to pathogens. Whether or not a pathogen is successful in causing disease is related to the health of the exposed individual and the state of his or her immune system, as well as to the number of pathogen cells required to make the person ill.
Some pathogens can cause disease when only a few cells are present.
In other cases, many cells are required to make a person ill. Third, it would be difficult to monitor for every possible pathogen. Each type of pathogen requires a specific test and many of these tests are time-consuming or expensive. Monitoring for each type of known pathogen would be prohibitively expensive. Finally, new pathogens are still being discovered.