1. Centers for Disease Control and Prevention, Division of HIV/AIDS Prevention, Atlanta, Georgia 30333.
2. Present address: Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, Michigan 48109-0620.
3. Department of Microbiology, Ullevaal University Hospital, N-0407 Oslo, Norway.
4. Norwegian Institute of Public Health, Oslo, Norway.
5. Laboratoire de Sante Hygiene Mobile, Ministry of Health, Yaounde, Cameroon.
6. Department of Hematology and Immunology, Faculty of Medicine, Yaounde, Cameroon.