RESEARCH Table 1. Recruitment and reasons for exclusion from study, United Kingdom (UK) Exclusion criteria Case-patients (n = 207) Refusal to participate Could not complete adequate interview Did not respond to letters or phone calls Not meeting the study case definition No history of diarrhea Mixed enteric infection Secondary case Travel outside the UK in 14 days before onset Travel in UK outside study area for >7 nights in the 14 days before onset Visitor to study area Residence outside study area Case-patient or household member previously interviewed as case-patient or control Total potential cases excluded Total cases enrolled Controls (n = 778) Refusal to participate in interview Unavailable at requested interview times Said interview times were not convenient Subtotal: refused, unavailable for interview Address not found History of diarrhea Travel outside UK in the 14 days before interview Travel in UK outside study area for >7 nights in the 14 days before interview Not resident in study area in the 14 days before interview Moved from study area Resident outside study area Control or household member already interviewed as a case-patient or control Subtotal: did not meet study control definition Interview cancelled by study team because three controls already enrolled for associated case Interview cancelled by study team as potential control found to be in wrong age group Reason for exclusion not recorded Subtotal: not enrolled for administrative reasons or reason not recorded Total potential controls excluded Total controls enrolled No. (%) excluded 1 (0.5) 1 (0.5) 2 (1.0) 1 (0.5) 1 (0.5) 36 (17.4) 8 (3.9) 1 (0.5) 1 (0.5) 1 (0.5) 2(1.0) 55 (26.6) 152 (73.4) 23 (3.0) 125 (16.1) 35 (4.5) 183 (23.5) 3 (0.4) 46 (5.9) 8 (1.0) 2 (0.3) 3 (0.4) 27 (3.5) 2 (0.3) 7 (0.9) 95 (12.2) 19 (2.4) 9 (1.2) 3 (0.4) 31 (4.0) 312 (40.1) 466 (59.9) 115 patients who had recovered when interviewed, the median duration of illness was 9 days (range 2–21). Nineteen (22.1%) of the 86 patients <5 years of age and 4 (8.5%) of 47 case-patients ages 6-15 years were admitted to hospital. Controls Three or four controls were recruited in association with 131 (86.2%) patients and one or two in association with the remainder. Patient and control groups were com- parable by sex, local government district of residence, water sources and water supply zones, disruption and dis- coloration of tap water, nights spent away from home with- in the United Kingdom in the 2 weeks before onset or interview, and sewage services to the home (Table 3). Time until Study Recruitment One hundred twenty-eight (84.2%) patients were inter- viewed within 1 week and 151 (99%) within 2 weeks of the date of the Cryptosporidium-positive fecal smear test report. The delay between reporting a case and enrolling the patient and associated controls was a median of 2.3 weeks (range 1–8). Knowledge about Cryptosporidium Thirty six (75%) of 48 patients and 113 (67.7%) of 167 controls recruited from April 29 to November 30, 1999, stated that they had not heard of Cryptosporidium before being contacted for the study. The proportion without knowledge was similar for patients and controls recruited before and after July 8, 1999. Of the 66 persons who had previously heard of Cryptosporidium, 16 had knowledge of modes of transmission: 4 reported transmission could occur through water, drinks, or contact with farms and ani- mals; 6 reported that transmission was by water only; and 6 reported transmission was by farm contact only. Single Variable Analysis Significant associations were seen with consuming cold unboiled tap water (odds ratio [OR] 2.12, 95% confidence interval [CI] 1.16 to 3.91, p = 0.012) with a significant dose-response relationship (χ2 test for trend p = 0.017). A significant dose-response relationship was also seen for the usual volume of cold unboiled tap water consumed at home (χ2 test for trend p = 0.005), but not for that con- sumed at the workplace, nursery, or school (χ2 test for trend p = 0.495) (online Appendix available from 1010 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004