 This study will last 1 year. Participants will be randomly assigned to one of two groups. At study entry, participants in each group will receive a single injection of either one of two doses of the adenoviral vector vaccine or placebo. Participants will record their temperature and other side effects in a symptom log on the day of vaccination and for 3 days thereafter. Participants will have seven clinic visits over 12 months. A physical exam, HIV and pregnancy prevention counseling, and blood and urine collection will occur at each visit. Participants will also be asked about side effects they may be experiencing and medications they are taking. 