We usually think that it is very difficult for the immune defenses to control HIV, the virus that causes AIDS, because the virus suppresses the immune response. However, recent research suggests that early on in an infection, a vigorous killer T-cell immune response keeps HIV in check. The problem is that this response weakens as time goes on because the immune cells become "exhausted." This killer T-cell "exhaustion" (actually down regulation in the language of modern biology) is a side effect of a normal mechanism that functions to protect against autoimmune disease. There are ways to re-energize exhausted killer cells, but these may pose risk to the patient. Rosenberg's work, however, points to a possible way to generate large numbers of highly active killer T-cells only aimed at targets infected with HIV, leaving normal tissues and organs unharmed.