The MTN-003 HIV prevention study include the use of microbicides, substances that kill microbes, and tenofovir disoproxil fumarate (TDF) and emtricitabine/tenofovir disoproxil fumarate (FTC/TDF) - oral, FDA-approved, anti-HIV drugs. The purpose of this study is to determine if taking daily TDF and FTC/TDF as a part of the study MTN-003 has an effect on bone mineral density (BMD).