RESEARCH Table 3. Baseline characteristics of study population Characteristics Sex M F Age, y 0–5 6–15 16+ Local government district Allerdale Copeland Source of water/zone Crummock Lake/ Crummock North Crummock Lake/ Crummock South Ennerdale Lake/ Ennerdale North Ennerdale Lake/ Ennerdale South Millom/Millom Quarry Hill/Quarry Hill Hausegill/Hausegill Hayknott/Hayknott Underscar/Underscar Fellside/Fellside Multiple sources/Mixed from >1 zone Different private water supplies Disruption of main supply in week before illness or outbreak? Y N Not recorded Water discolored in the week before illness or outbreak? Y N Not recorded Away from home in the 2 weeks before illness or outbreak? Y N Sewage services to home Mains Septic tank Chemical toilet Not recorded Cases (%) 152 (100) Controls (%) 466 (100) 82 (53.9) 70 (46.1) 86 (56.6) 47 (30.9) 19 (12.5) 83 (54.6) 69 (45.4) 37 (24.3) 19 (12.5) 29 (19.1) 13 (8.6) 19 (12.5) 16 (10.5) 3 (2.0) 2 (1.3) 1 (0.7) 0 11 (7.2) 2 (1.3) 8 (5.3) 143 (94.1) 1 (0.7) 14 (9.2) 130 (85.5) 8 (5.3) 49 (32.2) 103 (67.8) 130 (85.5) 21 (13.8) 1 (0.7) 0 236 (50.6) 230 (49.4) 273 (58.6) 136 (29.2) 57 (12.2) 248 (53.2) 218 (46.8) 103 (22.1) 49 (10.5) 101 (21.7) 43 (9.2) 54 (11.6) 41 (8.8) 5 (1.1) 6 (1.3) 3 (0.6) 1 (0.2) 51 (10.9) 9 (1.9) 14 (3.0) 448 (96.1) 4 (0.9) 33 (7.1) 431 (92.5) 2 (0.4) 163 (35.0) 303 (65.0) 411 (88.2) 51 (10.9) 0 4 (0.9) p value 0.539 0.904 0.838 0.941 0.294 0.398 0.603 genotype 2 (animal and human) strain, consistent with farmed livestock’s being a major source of infection. Risk was not increased by contact with pets. Excluding study participants by recent travel ensured that environmental exposures most likely occurred within the study area. Excluding household contact with an earli- er onset case ensured that person-to-person transmission within the household was unlikely to have occurred in study case-patients. The number of patients who sought medical attention was likely to have been high because the National Health Service provided free medical care within the study area at the point of use and because all fecal specimens were tested for Cryptosporidium by National Health Service laboratories without charge. Although only half of patients with cryptosporidiosis may seek medical atten- tion in the United Kingdom (23), we have no reason to suppose that risk factors for patients who do not visit healthcare facilities would differ substantially from those that did. Matching refers to pairing cases with one or more con- trols on the basis of their similarity in selected variables, with the objective of eliminating bias (24,25). We under- took stratified random sampling from a population list to select potential controls and adjusted by using multiple regression analysis, which is one of a number of alterna- tive designs to matching (24,25). Refusal to participate was low at 4 (1.9%) of 205 cases and 183 (23.5%) of 778 potential controls. Lower response rates in controls compared to cases is expected (24). Care was taken during the design and conduct of the study to mask interviewers to the tap water hypothesis. Interviewer training emphasized that all risk factors were plausible and required equal care in measurement. A survey after media 1012 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 6, June 2004