RESEARCH Medical and diagnostic microbiology services for Allerdale and Copeland residents were provided free at the point of use by the United Kingdom National Health Service and managed by North Cumbria Health Authority (14). The Authority was also responsible for maintaining the computer patient register and updating it for births, deaths, migration, and surveillance and control of infec- tious disease (14). The register held persons’ name, sex, date of birth, home address, and postal (zip) code but not medical information. The postal codes were geographical- ly referenced but did not share boundaries with public water supply distribution zones, government boundaries, or census enumeration districts. Methods Local ethical committees approved the protocol, and a study center was set up at North Cumbria Health Authority. All fecal specimens were examined for Cryptosporidium oocysts, regardless of whether this test was requested by the clinician. Family physicians and hos- pital clinicians were informed of the study, but not the main hypothesis, and were reminded to ensure best prac- tice in investigating cases of diarrhea with the assistance of the three local microbiology laboratories. Laboratory staff was requested to immediately report Cryptosporidium- positive fecal smear results to the study coordinator by telephone. Epidemiology Five workers were trained to conduct the study in a standard manner by using written protocols and question- naires. Both methods were pilot tested and their techniques refined before enrollment (15). Definitions Case-patients were defined as residents of Allerdale or Copeland with 1) diarrhea (three or more loose stools in a 24-hour period), 2) onset from March 1, 1996, to February 29, 2000, and 3) a fecal smear positive for Cryptospori- dium oocysts but negative for other enteric pathogens. Patients were excluded if they had, within 14 days of onset of illness, contact with another household member with cryptosporidiosis or any diarrhea illness, traveled outside the United Kingdom, stayed away from home outside the study area within the United Kingdom for >7 nights, or if they or a household member had already been enrolled as a patient or control at any time during the study. Controls were defined as residents of Allerdale or Copeland with no history of diarrhea (three or more loose stools in a 24-hour period) in the 14 days before interview. Potential controls were excluded if they had, within 14 days of interview, traveled outside the United Kingdom, stayed away from home outside the study area but within the United Kingdom for >7 nights, or if they or a house- hold member had already been enrolled as a patient or con- trol at any time during the study. Three controls were randomly selected, by using a computer algorithm, from subtables of the health authority patient register with the same age span (0–5, 6–15, and 16+ years) and the same lead characters of the postal code as the case-patient. The process was repeated when neces- sary to replace persons who declined, could not be contact- ed, or met exclusion criteria. The patient register was compared with population estimates from the 2001 census. Interviews Interviews were conducted face-to-face at home. Participants <18 years of age were interviewed with par- ents or guardians, who acted as proxies for younger chil- dren, as appropriate. Informed consent was obtained and recorded. Case-patients and associated controls were enrolled as close to each other in time as possible. For unboiled cold tap water, bottled water, and soft drinks, we asked about “usual” consumption without a time restriction. For all remaining exposures, including consuming pasteurized and unpasteurized milk, ice, and foods, exposure was sought for the 2 weeks before onset of illness for patients and before date of interview for con- trols. The volume of different fluids drunk by study partici- pants was determined by showing a standard picture card of a glass, cup, and mug; the volume was recorded accord- ing to the calibration on the card. The usual daily volume of water as cold unboiled tap water drunk at home was determined as water alone and as a diluent in cold fruit- squash type drinks. The usual daily volume of cold unboiled tap water drunk at work, school, or nursery at locations within Copeland and Allerdale District bound- aries was determined the same way. The two usual daily volumes consumed at and away from home were added to give the total usual daily volume of cold unboiled tap water consumed within Copeland and Allerdale Districts. The usual daily volume consumed of bottled drinking water and soft drinks not diluted with water was deter- mined separately without distinction between consuming it at home or away from home. Questions about consumption of ice in cold drinks were asked separately for ice made at home and ice consumed at work, school, or nursery. The following types of contact were recorded: farms, farmed animals, and handling and feeding farm animals; slurry; household pets and feeding household pets; chil- dren’s nurseries; and recreational exposure to water in swimming pools, rivers, and streams. Information about consumption and frequency of consumption of uncooked salad items, uncooked meat, uncooked sausage meat and sausages, yogurt, cheese, and cream was also elicited. 1008 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004