Emerging Infectious Diseases

diarrheal illness in an estimated 403,000 persons (1). Following that outbreak, testing for Cryptosporidium in persons with diarrhea increased substantially in some areas of Wisconsin; by August 1, 1993, three of six clinical laboratories in Dane County were testing routinely for Cryptosporidium as part of ova and parasite examinations. In late August 1993, the Madison Department of Public Health and the Dane County Public Health Division identified two clusters of persons with laboratory-confirmed Cryptosporidium infection in Dane County (approximately 80 miles west of Milwaukee). This report summarizes the outbreak investigations. On August 23, a parent reported to the Madison Department of Public Health that her daughter was ill with laboratory-confirmed Cryptosporidium infection and that other members of her daughter’s swim team had had severe diarrhea. On August 26, public health officials inspected the pool where the team practiced (pool A) and interviewed a convenience sample of patrons at the pool. Seventeen (55%) of 31 pool patrons interviewed reported having had watery diarrhea for 2 or more days with onset during July or August. Eight (47%) of the 17 had had watery diarrhea longer than