Microorganisms of public health concern in water can be controlled by treatment process technology currently available. Where outbreaks have occurred, the cause has been a demonstrated failure in wastewater or water supply treatment operation or distribution protection. While natural self-purification in receiving waters can be an important buffer between waste effluent discharges and water supply intakes, the fragile nature of the associated phenomena cannot be a substitution for adequate waste treatment. Essential to maximum public health safeguards is the use of a multiple barrier concept that relies on establishing protective systems between the water user and sources of potential contamination.