Abstract
An outbreak of cryptosporidiosis which affected 44 people in January and February 1991 was identified through local surveillance at a South London Public Health Laboratory. Preliminary enquiries revealed that more than half the patients were adult and that there were no common factors other than geographical association. A case-control study showed a significant association between illness and consumption of tap water supplied by a particular water company, as well as a dose response effect. There were no apparent breaches or irregularities in the water distribution system and no indication of a problem through routine monitoring indices. This incident demonstrates the problems of establishing the source of cryptosporidiosis outbreaks in the absence of evidence of environmental abnormality, as well as possibly indicating that water conforming to current treatment standards may occasionally contain sufficient numbers of Cryptosporidium oocysts to cause sporadic cases or clusters.