Since the 19th century, faecal bacterial indicators have been used to detect the presence of faecal contamination in water and to infer the possible presence of waterborne pathogens. However, many outbreaks caused by viral pathogens transmitted in bacteriologically safe waters showed the need for improved water quality monitoring with faecal viral indicators. The answer to this question is coliphages, viruses that infect enterobacteria, mainly Escherichia coli but not exclusively.
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