1. Department of Epidemiology, Colorado School of Public Health, University of Colorado Denver, Aurora, Colorado.
2. Enteric Diseases Epidemiology Branch, Division of Foodborne, Waterborne, and Environmental Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia.
3. Biostatistics and Information Management Office, Division of Foodborne, Waterborne, and Environmental Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia.