Tenofovir disoproxil fumarate (also known as tenofovir DF or Viread) is used with other anti-HIV drugs to treat HIV infection. Taking tenofovir DF every day may also prevent HIV infection. This study will determine if taking a tenofovir DF tablet every day is safe and effective in preventing HIV infection. Participants in the study will be sex workers in Phnom Penh, Cambodia.