Evaluation of the Cellular Pharmacology of Tenofovir and Emtricitabine According to HIV Infection Status

Inclusion Criteria for HIV-Uninfected Participants: * Ability to provide informed consent * Ability to comply with the study procedures Exclusion Criteria for HIV-Uninfected Participants: * Positive screening test for HIV infection * Positive screening test for hepatitis B (HBV) infection * Pregnant or planning to become pregnant in the 3 months after study entry * Breastfeeding * If sexually active and fertile (no tubal ligation or hysterectomy), refusal to use two forms of birth control (e.g., condom and hormonal birth control) during the 60-day study * Estimated glomerular filtration rate (GFR) less than 60 mL/min/1.73 m\^2 by the Modification of Diet in Renal Disease (MDRD) method * Albuminuria (greater than 30 mg urine albumin per g of urine creatinine) * Blood donation within 56 days of the screening visit * Any grade I or higher abnormality in hemoglobin, platelets, serum phosphorous, and lipase on the screening visit; grade I abnormalities in other labs will be evaluated on a case by case basis (using DAIDS criteria) * Any greater than grade I abnormality in screening laboratory tests (using DAIDS grading criteria) * Medical history of chronic uncontrolled high blood pressure equal to or above 140/90 mm Hg * Use of any investigational medication in the 30 days before study entry * Daily anticoagulant therapy (daily aspirin or non-steroidal anti-inflammatory drugs \[NSAIDs\] will be allowed if discontinued for 1 week prior to the rectal biopsy) * Any nephrotoxic concomitant medication (e.g., aminoglycosides, cyclosporine, cidofovir, foscarnet, amphotericin B) * Active recreational drug or alcohol abuse * Any concomitant medication (or herbal product) that, in the opinion of the investigators, would interfere with the study outcomes (acceptable medications include acetaminophen, occasional ibuprofen/NSAID, vitamins, and birth control pills) * History of pathologic bone fractures * Any chronic or acute medical condition that, in the opinion of the investigator, would interfere with study conditions, such as cancer, heart disease, or diabetes * Body weight under 110 pounds Inclusion Criteria for HIV-Infected Participants: * HIV-infected adults (HIV documented in medical record or by the primary clinician) * Clinician/participant plan to initiate TDF/FTC/EFV therapy and agree to separate TDF/FTC and EFV prescriptions for the initial 30 days of the study * Ability to provide informed consent * Ability to comply with the study procedures Exclusion Criteria for HIV-Infected Participants: * Antiretroviral therapy in the preceding 6 months * Pregnant or planning to become pregnant in the 3 months after study entry * Breastfeeding * If sexually active and fertile (no tubal ligation or hysterectomy), refusal to use two forms of birth control (e.g., condom and hormonal birth control) during the 60-day study * Estimated GFR less than 60 mL/min/1.73 m\^2 by the MDRD method * Albuminuria (greater than 30 mg urine albumin per g of urine creatinine) * Greater than a grade II abnormality in hemoglobin or platelets. Greater than a grade II abnormality in other clinical chemistry or hematology tests that, in the opinion of the investigators (principal investigator, study coordinator, and study physician) and primary clinician, would preclude participation in the study. DAIDS grading criteria will be used. * Use of any investigational medication in the 30 days before study entry * Daily anticoagulant therapy (daily aspirin or NSAIDs will be allowed if discontinued for 1 week prior to the rectal biopsy) * Any nephrotoxic concomitant medication (e.g., aminoglycosides, cyclosporine, cidofovir, foscarnet, amphotericin B) * Any concomitant medication (or herbal product) that, in the opinion of the investigators, would interfere with the study outcomes (acceptable medications include acetaminophen, occasional ibuprofen/NSAID, vitamins, and birth control pills) * Any chronic or acute medical condition that, in the opinion of the investigator, could lead to emergent health complications, or could interfere with the participant's ability to follow study procedures * Body weight under 110 pounds