1. From the Department of Epidemiology and Virology, Baylor College of Medicine, the Department of Medicine, Washington University School of Medicine, the Department of Medicine, UCLA Center for the Health Sciences, and the Blood Resource Branch, National Institutes of Health (address reprint requests to Dr. Roche at the National Institutes of Health, Bldg. 31, Room 4A–03, Bethesda, Md. 20014).