Microbicides are topical medicines that can prevent infection by Human Immunodeficiency Virus (HIV). Microbicide medicine has yet to be studied in adolescents, a key group that is becoming infected with HIV all over the world. From past research, we know that at different ages people experience age-related changes in their bodies that can cause differences in how they process medications. In this study, gut tissue samples (or gut biopsies) from 12 HIV-negative volunteers will be collected. These pieces of tissue will be infected with HIV in the laboratory to develop a model that can be used to test certain drugs against the HIV infection. We can use this tissue to test a drug called tenofovir against HIV infection. We will determine whether this drug can decrease HIV infection in the gut biopsies. In this study, we will also measure HIV levels and the levels of tenofovir in gut and blood samples in 12 people who are already taking this drug. This information can determine whether levels of drug found in the gut can protect it from HIV. The results can be compared to other age groups of adolescents and adults. Subjects will undergo a common procedure called a lower endoscopy (this can be a colonoscopy or a flexible sigmoidoscopy) to obtain gut biopsy samples.