Cryptosporidium, a protozoan parasite, has emerged during the 1980s as a common cause of gastroenteritis in otherwise healthy subjects and of potentially life-threatening infection in the immunocompromised. The parasite, a member of the coccidia, has a complex life-cycle resulting in the production of an environmentally hardy stage, the oocyst, excreted in the faeces. Cryptosporidium parvum infects a wide range of host species including man and his livestock animals. The epidemiology is complex with both direct and indirect routes of transmission. Environmental contamination may result in dissemination of the infection by the water route. Such water may meet current microbiological and other standards. The oocysts are remarkably resistent to most disinfectants including chlorine but sensitive to ozone. Water may provide the vehicle of infection for sporadic cases and outbreaks, some involving thousands of consumers. Such outbreaks have been identified in the USA and in the UK.

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