The past five years have seen remarkable progress in both treatment and basic understanding of the human immunodeficiency virus (HIV) that causes AIDS. In laboratories and clinics across the country and around the world, scientists and doctors have pooled their expertise to keep people infected with HIV alive and disease-free longer than was imaginable at the start of the epidemic. And now, through what may well be an unprecedented cross-fertilization process among molecular biologists, immunologists, geneticists, and practicing physicians, a series of discoveries about HIV-infected patients who have successfully fought off AIDS for as long as 20 years are being closely analyzed for clues to the ultimate goal in this fierce scientific battle—a vaccine. NOVA tells the story of this ongoing battle through the experiences of patients.