Water intended for public use is examined for an indication of fecal contamination. When such indication is found, it is assumed that the water is potentially dangerous.
The reason for this is the direct search for the presence of a specific pathogen in water is too uneconomical, time consuming and unwieldy for routine control purposes. Inasmuch as the bacterial flora vary from one source of supply to another, standards must not be interpreted too rigidly. A recommended practice is to rate each individual supply on the basis of frequent examinations and to suspect at once any deviation from the usual quality of the natural water supply.