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).