Tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) are two antiretroviral medications used for the treatment and prevention of HIV/AIDS. This study will examine how these medications are processed in the body of people who are HIV-infected, as well as in people who are HIV-uninfected.

Tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) are both nucleoside reverse transcriptase inhibitors (NRTIs), a class of medications used for the treatment and prevention of HIV/AIDS. Analyzing how the body interacts with these medications at the cellular level may lead to more effective dosing strategies for both HIV prevention and treatment. This study will examine the pharmacokinetics of TDF and FTC at the cellular level in HIV-infected people (N=20) and HIV-uninfected people (N=20). HIV-infected participants will be allowed to take part in this study only if their doctor already plans to prescribe TDF, FTC, and efavirenz (EFV) for their HIV care, regardless of their participation in this study. HIV-infected participants will receive Truvada (TDF/FTC) and EFV for the first 30 days. After Day 30, participants will continue to receive TDF, FTC, and EFV through Day 60, under the direction of their physician. HIV-infected participants will remain on their therapy throughout the study as part of their HIV care. HIV-uninfected volunteers will receive 30 days of Truvada (TDF/FTC).

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).

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