The nature of the water supply (public or private) and sewage services (public or septic tank) to the home were recorded and corroborated against water company records. Disruption to tap water or change in tap water color or taste in the week before onset of illness for patients, and in the week before date of interview for controls, was record- ed. Information on sources, water treatment works, and the water that supplied each address’s postal code was obtained from water company records and linked to indi- vidual patient and control records. Knowledge about Cryptosporidium Infection A series of television and newspaper articles on Cryptosporidium occurred in North Cumbria in April and May 1999. Study participants enrolled from April 28 to November 30, 1999, were also asked if they had heard of Cryptosporidium and what they knew about it. Data Entry and Analysis Data were double entered and differences edited and corrected in EpiInfo version 6.0.4.d. (Centers for Disease Control and Prevention, Atlanta, GA). Fluid intakes were analyzed as yes/no responses; within categories (<1/4 pint, 1/4–1 pint, >1–2 pints, and >2 pints) with χ2 tests for trend in single variable analysis; and as actual volume consumed in multivariable analysis (16). Variables positively associated with infection at the p < 0.2 level in the single variable analysis, age, sex, and water supply zone were included in the initial multivariable model. Backward stepwise logistic regression was under- taken by comparing nested models using Likelihood Ratio Tests and GLIM software (16,17). Variables with signifi- cance p > 0.3 in iterations of the multivariable model were removed in stepwise fashion except for age, sex, and water supply zones, which were retained in all models regardless of their significance. Subsidiary analyses modeled usual daily consumption of tap water at home and usual total daily tap water consumption; cases and controls served by mixes of water from more than one source were omitted. Microscopy of stained fecal smears for Cryptospori- dium oocysts (18), Giardia, and culture for pathogenic enteric bacteria were undertaken by using standard meth- ods at each of the three local microbiology laboratories. Smears in which Cryptosporidium oocysts were identified from January 1998 to February 2000 were also analyzed by polymerase chain reaction and restriction fragment length polymorphism typing of a region of the Cryptosporidium oocyst wall protein gene (19,20). Results None of the incident cases arising from the study pop- ulation during the 4 years of the study were linked or clus- tered in time and space. All were considered sporadic Sporadic Cryptosporidiosis, England, 1996–2000 infections eligible for inclusion in the study. No changes occurred in livestock farming, livestock densities, water sources, or water treatment within the study area during the study period. Potential Study Cases and Exclusions Two hundred seven case-patients were ascertained dur- ing the study period; 152 (73.4%) were enrolled, and 55 (26.6%) were excluded (Table 1). One refused to partici- pate, one was unable to complete the interview, and two did not respond. Thirty-six (17.4%) were secondary to a laboratory-confirmed case in the household, 8 (3.9%) had traveled outside the United Kingdom, and 1 (0.5%) had traveled within the United Kingdom and stayed outside the study area >7 nights. Further single case-patients were excluded for having no history of diarrhea, mixed enteric infection, and being a visitor to, or resident outside, the study area. Two additional case-patients were excluded because a member of their household had already been interviewed as a case-patient or control earlier in the study. Potential Controls and Exclusions Seven hundred seventy-eight potential controls were identified; 466 (59.9%) were enrolled, and 312 (40.1%) were excluded (Table 1). One hundred eighty-three (23.5%) refused to participate or were unavailable for interview. The address of three (0.4%) could not be found. Forty-six (5.9%) had a history of diarrhea, 8 (1.0%) had traveled outside the United Kingdom, and 2 (0.3%) had traveled within the United Kingdom away from the study area for >7 nights in the 2 weeks before interview. Twenty- seven (3.5%) had moved from the study area, 2 (0.3%) had an address outside the study area, and 7 (0.9%) shared a household with a patient or a control. Nineteen (2.4%) were not enrolled because 3 controls had already been recruited in association with the case, 9 (1.2%) were in the wrong age band, and 3 (0.4%) had no reason recorded. Study Population Cases Of the 152 study case-patients, 86 (56.6%) were <6 years of age; 47 (30.9%) were 6–15; and 19 (12.5%) were >16 years. Eighty-two (53.9%) were male. More cases were detected in the first half of each year of the study (Figure). The average annual incidence rate was similar in populations served by water from Crummock Lake, Ennerdale Lake, and the other water sources combined (Table 2). In addition to diarrhea, the 152 patients reported abdominal pain (110 [72.4%]), vomiting (94 [61.8%]), fever (69 [45.4%]), anorexia (68 [44.7%]), and weight loss (56 [36.8%]). Thirty-seven patients were ill at interview. In Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004 1009