I don't think they were designed for that purpose

I don't think they were designed for that purpose," says Gallo. "It's a stroke of luck and a stroke of nature." In 1995, his team discovered that certain chemokines can physically block the receptor proteins on the surface of cells that HIV uses to infect them. Their quest now is to see if there may be a way to replicate this blockade in drug form. The hope is that a new class of drugs to intervene against HIV might prove successful with patients who cannot tolerate the current anti-viral "cocktails," or might offer a less expensive way to keep the virus at bay without years of ongoing treatment.

Dr. Norman Letvin

Many scientists, meanwhile, are turning their attention toward the idea of a "therapeutic" vaccine—one that would not prevent AIDS infection but would keep an infected individual from succumbing to the ravages of the disease. This new approach is partly the result of the realization after years of effort that a completely preventive vaccine for a virus as devious as HIV might prove impossible.

"Making a vaccine to prevent HIV infection is orders of magnitude more difficult than a vaccine to prevent other infectious diseases we've conquered in the past," says Dr. Norman Letvin of Beth Israel Deaconess Medical Center in Boston, who is a member of the National Institutes of Health's AIDS Vaccine Development Advisory Committee. "Other vaccines have been successful because they have not had to generate what we would call sterilizing or absolutely protective immunity."

A good example of a vaccine that doesn't actually prevent infection with the virus but keeps the individual from contracting disease is the polio vaccine, Letvin says. The polio virus enters the body and replicates initially in the blood. It eventually makes its way to the spinal cord, where it causes disease. "The polio vaccine doesn't prevent the polio virus from entering the blood," Letvin says. "Rather it slows down the replication of the virus just enough to prevent it from entering the spinal cord and causing neurologic disease."