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.