To describe the magnitude and duration of changes in HIV-1 RNA levels during and after an acute febrile illness. To identify factors associated with increases, i.e., type of illness ultimately diagnosed (bacterial, viral, fungal), CD4 cell count, and antiretroviral treatment regimen. To describe changes in phenotypic markers of immune activation/dysregulation of CD4 and CD8 lymphocyte subsets and their relationship to intercurrent illness. To describe changes in plasma cytokines and soluble activation markers and their relationship to plasma HIV-1 viremia during and after the onset of intercurrent illness. To characterize the viral biologic phenotype and the viral drug susceptibility genotype before, during, and after the onset of an acute febrile illness. To characterize the expression of HIV-1 co-receptors before, during, and after the onset of an acute febrile illness Repeated episodes of intercurrent infections have been postulated to be an important stimulus for progression of HIV infection. The study of intercurrent illness in patients with initially undetectable viral load removes viral load as a possible cause for virologic and immunologic changes and allows for a more direct association of the intercurrent illness with changes in viral load, viral HIV-1 phenotypes, viral HIV-1 genotypes, and T cell phenotypes. Studying intercurrent illness and viral load provides an opportunity to characterize the potentially dynamic changes not only in viral load but also in phenotypic markers of T cell activation, plasma cytokine levels, phenotypic and genotypic changes in circulating virus, and HIV-1 tropisms.