1. From the Department of Epidemiology, The Johns Hopkins University School of Hygiene and Public Health, Baltimore, Md (F.J.N., G.W.C., M.S.), National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md (P.S.), Division of Hematology, University of Texas Medical School, Houston, Tex (K.W.), and the Division of Molecular Virology, Baylor College of Medicine, Houston, Tex (E.A., J.L.M.).