Hopes of finding a “magic bullet” to stop AIDS have gradually faded over the years, as one promising drug after another has fallen short of the target. All four antiviral drugs approved for use in the United States eventually lose their ability to fight the AIDS virus. And development of a vaccine to prevent infection has proven even more frustrating. But the pall of discouragement that has settled over AIDS research may be lifting. New insights into the murky realms of HIV biology and the human immune system may lead to more effective drugs or vaccines. Meanwhile, researchers' initial focus on finding a quick treatment is giving way to a growing consensus that they must learn more about how the virus works before they can find a cure.