“…Advanced waste water treatment methods, like the activated sludge process, may greatly reduce chemical pollution, but potential microbial pathogens may persist (Berg, 1973;Clarke & Chang, 1959). However, it has been suggested that low-level circulation of pathogens in our water systems may assist in the maintenance of general immunity (Chang, 1968; Mosley, 1967), although it is more generally agreed that pathogen-free supplies are desirable.…”