The central hypothesis is that tissue drug profiles of tenofovir (TFV) and its active component, tenofovir disoproxil fumarate (TDF), and tissue infectibility vary between younger (10-14 years old) versus older adolescents (18-21 years old), and that both differ from adults (>21 years). Specifically, younger HIV positive adolescents will have lower levels of tissue tenofovir compared to older HIV positive adolescents and adults in an age-dependent manner. Additionally, biopsies from younger HIV negative adolescents will have: 1) higher rates of infection compared to biopsies from older HIV negative adolescents infected with a lower dose of virus; and 2) lower percent suppression of tissue infectivity compared to biopsies from older HIV negative adolescents using low dose tenofovir.