In the 20years since the first cases of AIDS were detected, scientists say they have learned more about this viral disease than any other.

Yet Peter Piot, who directs the United Nations AIDS program, and Stefano Vella of Rome, president of the International AIDS Society, and other experts say reviewing unanswered questions could prove useful as a measure of progress for AIDS and other diseases.

Among the important broader scientific questions that remain：

Why does AIDS predispose infected persons to certain types of cancer and infections？

A long-standing belief is that cancer cells constantly develop and are held in check by a healthy immune system. But AIDS has challenged that belief. People with AIDS are much more prone to certain cancers like non-Hodgkins lymphomas and Kaposi’s sarcoma , but not to breast, colon and lung, the most common cancers in the United States. This pattern suggests that an impaired immune system, at least the type that occurs in AIDS, does not allow common cancers to develop.

What route does HIV take after it enters the body to destroy the immune system？

When HIV is transmitted sexually, the virus must cross a tissue barrier to enter the body. How that happens is still unclear. The virus might invade directly or be carried by a series of different kinds of cells.

Eventually HIV travels through lymph vessels to lymph nodes and the rest of the lymph system. But what is not known is how the virus proceeds to destroy the body’s CD-4cells that are needed to combat invading infectious agents.

How does HIV subvert the immune sys-tem？

Although HIV kills the immune cells sent to kill the virus, there is widespread variation in the rate at which HIV infected people become ill with AIDS. So scientists ask：Can the elements of the immune system responsible for that variability be identified？If so, can they be used to stop progression to AIDS in infected individuals and possibly prevent infection in the first place？

What is the most effective anti-HIV therapy？

In theory, early treatment should offer the best chance of preserving immune function. But the new drugs do not completely eliminate HIV from the body so the medicines, which can have dangerous side effects, will have to be taken for a lifetime and perhaps changed to combat resistance. The new policy is expected to recommend that treatment be deferred until there are signs the immune system is weakening.

Is a vaccine possible？

There is little question that an effective vaccine is crucial to controlling the epidemic . Yet only one has reached the stage of full testing, and there is wide controversy over the degree of protection it will provide. HIV strains that are transmitted in various areas of the world differ genetically. It is not known whether a vaccine derived from one type of HIV will confer protection against other types.

In the absence of a vaccine, how can HIV be stopped？

Without more incisive , focused behavioral research, prevention messages alone will not stop the global epidemic.