1. Department of Epidemiology, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7435
2. Department of Pediatrics and Microbiology and Immunology, Elizabeth B. Lamb Center for Pediatric Research, Vanderbilt University Medical Center, Nashville, Tennessee 37232-2581
3. Department of Microbiology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6076
4. Department of Microbiology and Immunology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7290