But Jerome Zack, an HIV researcher at the

But Jerome Zack, an HIV researcher at the University of California at Los Angeles, told MSNBC that "you can't eliminate the potential for there still being low-level virus in the body that's undetectable."

There's also a problem with replicating the same procedure in otherwise healthy patients who are HIV-positive but don't have leukemia. Plus, the genetic mutation found in Brown's donor, which makes patients resistant to HIV, is extremely rare.

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"This probably is a cure, but it comes at a bit of a price," Dr. Michael Saag, professor of medicine and director of the University of Alabama at Birmingham AIDS Center, told CNN.

"For him to receive the donor cells, his body had to have all of his immune system wiped out" and then receive a bone marrow transplant, Saag said. "The catch-22 here is that the best candidates for a cure, ideally, are people who are healthy" and don't have leukemia, he said.

Stem cell transplants are very risky because they wipe out the patient's immune system to the point where they could die without an insertion of healthy donor cells. That process is "very hazardous," Saag said. "Even if somebody doesn't die from a transplant, there are complications that make it very unpleasant for people to live with."