1. Department of Pathology, The Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
2. Statistical Center for HIV/AIDS Research and Prevention, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109.
3. Department of Biostatistics, University of Washington, Seattle, Washington 98195.
4. Department of Paediatrics, Makerere University, Kampala, Uganda.
5. Department of Obstetrics and Gynaecology, Makerere University, Kampala, Uganda.