1. Department of Epidemiology, Johns Hopkins University School of Public Health, Baltimore, Maryland 21205.
2. Comprehensive AIDS Center, Northwestern University Medical School, Chicago, Illinois 60611
3. Department of Public Health, UCLA School of Public Health, Los Angeles, California 90095
4. Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
5. Department of Molecular Microbiology and Immunology, Johns Hopkins University School of Public Health, Baltimore, Maryland 21205