Volunteers are recruited from Malawi, South Africa, Zambia, and Zimbabwe. The earliest possible cases of seroconversion are included. At enrollment, participants are counseled appropriately for their HIV status and demographic information is obtained. Participants are followed quarterly up to 12 months. Epidemiological, clinical, and laboratory data are collected during physical exams and blood draws at each visit. Blood samples are used to assess CD4 counts, plasma viral load, genetic parameters, and individual immune responses. Participants who are seronegative or whose status is unknown are tested for HIV at each visit, with post-test counseling when participants return to the clinic for test results.