More than 30 years after scientists identified HIV as the cause of AIDS, we still have not managed to devise an effective vaccine against the virus. An array of drugs can usually keep the infection under control for decades, but a vaccine that prevents infection in the first place would be the best weapon—particularly in the developing world, where the costs of drugs and other factors can put effective therapy beyond the reach of many. Without treatment, HIV infection usually runs silently and progresses to severe immunodeficiency (AIDS) and death over the course of several years.