1. Division of Clinical Epidemiology, University of Utah School of Medicine, Salt Lake City, Utah.
2. Minneapolis VA Medical Center and Department of Medicine, University of Minnesota, Minneapolis, Minnesota.
3. Foodborne and Diarrheal Diseases Branch, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia.
4. VA Salt Lake City Health Care System, Division of Clinical Epidemiology, University of Utah School of Medicine, Salt Lake City, Utah.