Thirty-two waterborne outbreaks in Sweden are known during 1975 - 1984, affecting nearly 12 000 people. These range from single family outbreaks to community outbreaks affecting up to 3 000 people. Microbial agents have been isolated in about 40 % of the outbreaks, the rest are of unknown etiology. Epidemiological investigations have shown that only a fraction of the actual number of cases are initially reported. The real number as judged from epidemiological follow-up investigations was in many instances tenfold higher. In almost all the cases, the cause of the outbreaks are technical deficiencies like back-siphonage of wastewater along drainage pipes, broken sewerage or sudden pollution of raw water intakes coinciding with malfunction of chlorination.
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Andersson et al. (1986) studied this question.