This is a study to determine whether patients exhibit a temporary burst of viral replication or other changes in response to intercurrent febrile illness. Although there is no study treatment, patients on this study must be co-enrolled in at least 1 other ACTG antiretroviral treatment study. Plasma HIV-1 RNA and other variables are measured at the time of presentation, on Day 3, and at Weeks 1, 2, 4, 8, 16, and 24.