(CBS) One of the greatest mysteries of the AIDS epidemic may be solved, CBS News correspondent Sharyn Alfonsi reports.

Two teams of scientists announced Monday that they know why the immune system is unable to control the HIV virus and, more importantly, how we - one day - might be able to manipulate it to fight everything from HIV to cancer.

A healthy body generates something called T-cells or "killer cells," which attack viruses. But what scientists like Bruce Walker didn't know, until now, is why those killer cells stopped working in most HIV patients.

"One hypothesis has been that they become inactivated. One hypothesis is that that they became destroyed," Dr. Bruce Walker says. "What this study shows us is actually that those cells are there, that they fully function; it's just that they have been turned off."

The cells are turned off by HIV, which disarms them by flicking off a molecular switch in the cells. But in the laboratory, researchers were able to block that switch and restore T-cell function.

The findings raise the possibility that one day, doctors could switch a chronically ill patient's immune system back "on" so that it could resume its fight against HIV, cancer or even Hepatitis C.

But there's one big concern. Manipulating the immune system could send it into overdrive, triggering an auto-immune disease, attacking healthy parts of the body.

"We really don't know what happens when we try this with humans," Walker says. "But it opens a new pathway for us to pursue. But we really need to proceed with caution."

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