The study duration is 60 days. Study visits will occur at baseline and on Days 1, 3, 7, 20, 30, and 60. At most study visits, participants will undergo blood and urine collection for pharmacology studies, a medication history review, and an adverse effects questionnaire. HIV-uninfected participants will also attend two additional study visits at Days 35 and 45 - while off study medication - for blood and urine collection, adverse effects questionnaires, and a medication history review. At varying study visits during the first 30 days, all participants will undergo one rectal biopsy, female participants will undergo one cervical cell and fluid sampling procedure, and male participants will provide one semen sample. In addition to the collections from enrolled participants, study researchers will also analyze previously collected and stored blood samples from participants in the "Chemoprophylaxis for HIV Prevention in Men (iPrEx)" study, which examined the use of TDF and FTC for the prevention of HIV in men who have sex with men (MSM).