The Compartmental Biology of HIV in the Male Genital Tract

8 HIV positive men taking TDF/FTC and 8 HIV negative men taking TDF/FTC as pre-exposure prophylaxis will provide multiple semen and blood samples during a 48-hour inpatient visit. 8 HIV positive men taking TAF (tenofovir alafenamide) will provide multiple semen and blood samples during a 48-hour inpatient visit. Participants will take part in the study for approximately two months. After the screening visit, there is one 2 day overnight visit for intensive PK/PD (pharmacokinetic/pharmacodynamic) sampling. The investigators will study drug concentrations and intracellular endogenous nucleotide concentrations (dATP and dCTP) in seminal plasma and (where appropriate) seminal cells. Samples will be analyzed through the use of novel laboratory methods to determine the seminal plasma and seminal cell concentrations of tenofovir and emtricitabine. New technologies will be used to better understand compartmental and intracellular antiretroviral pharmacology of nucleoside/tide reverse transcriptase inhibitors. Pharmacokinetic modeling will be used to estimate the primary outcomes.