Why is it so hard to control STDs?

At the 2007 International AIDS Society in Sydney according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, "Three years ago, fewer than 300,000 people in the developing world were receiving the anti-retroviral drugs that help treat the virus. Last year, 2.2-million people in developing countries received the drugs. However, for every one person that you put in therapy, six new people get infected."