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Historical public health concerns about the transmission of communicable disease through drinking water have returned to prominence since the emergence of new organisms, such as Cryptosporidium parvum, which initially eluded twentieth century water treatment procedures. Rates of cryptosporidiosis are high in New Zealand by international standards. Collation of national data on communicable disease and drinking water quality has shown that residents of small communities do not enjoy the same quality of drinking-water as their counterparts in major urban centres, and they also experience higher reported rates of cryptosporidiosis. Drinking water quality is associated with socio-economic deprivation at small area level. Aspects of our freshwater ecology, and the ubiquitous use of surface water drinking supplies, may contribute to high rates of disease. Individual level data are important to further our understanding of the relationship between people, drinking water supplies, and patterns of disease transmission in the community. Therefore, we sought to identify risk factors for exposure to cryptosporidiosis among blood donors in New Zealand, and test those donors for serological evidence of exposure to Cryptosporidium parvum. Methods Cross-sectional anonymous self-completed questionnaire survey of blood donors in two small New Zealand towns, and serological investigation of antibodies to Cryptosporidium parvum. Results Ninetynine percent of eligible donors agreed to participate. Exposure to possible risk factors was common and included drinking unboiled tap water, contact with preschool children, contact with household and farm animals, travel to other urban and rural areas within New Zealand, swimming in public pools or fresh water, and travel overseas. One fifth of respondents in community A reported high risk occupations which involved exposure to young children, farm animals, hospital inpatients, or bio-specimens. Community B had a drinking water supply with an unsatisfactory public health grading. Almost one third of respondents reported illness with diarrhoea and/or vomiting in the previous year; only 12% of these participants sought medical attention for the illness and only 7% had stool samples taken. Seroprevalence was the same in the two communities, even when controlling for different exposure to risk factors. Over 60 per cent of participants showed antibodies to either the 17-kDa (63.9%; 95%CI 58.6–69.2) or 27-kDa (64.2%; 95%CI 59–69.4) Cryptosporidium parvum antigen groups. Half the participants had antibodies to both antigen groups (49.5%; 95%CI 44–55). Conclusion Poor drinking water quality in New Zealand is associated with high rates of cryptosporidiosis, and with small area socio-economic deprivation. At an individual level New Zealanders are exposed to multiple risk factors for infection with Cryptosporidium parvum. The high rates of seropositivity suggest a degree of immunity in the adult population, which may not be shared by children who are more susceptible to symptomatic infection. Improvement of drinking water quality remains an issue of social equity with the potential to reduce disproportionate burdens of disease.