1. Harvard School of Public Health AIDS Initiative, Harvard School of Public Health, Boston, Massachusetts 02115.
2. Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, Massachusetts 02115.
3. Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts 02115.
4. Interdisciplinary Program in Infectious Disease Epidemiology, Harvard School of Public Health, Boston, Massachusetts 02115.