Doctors in Germany said they have likely cured an American's HIV infection after implanting him with bone marrow stem cells bearing an uncommon genetic mutation making the carrier nearly immune to the virus.

But scientists caution that the difficult, dangerous procedure could be tough to replicate.

Three years ago, Timothy Ray Brown, who was HIV-positive, received a stem cell transplant to treat his leukemia. For the procedure, first his immune system was wiped out with radiation and chemotherapy, and then he was given bone marrow stem cells from a matching donor to build it back up.

However, the doctors found a donor who inherited a rare HIV-resistant mutation from both parents. About 1 percent of Europeans inherit this mutation from both parents; it's almost nonexistent in other groups.

Three years later, the patient's recovering from the leukemia treatment and is HIV-negative, the researchers said.

"[O]ur results strongly suggest that cure of HIV has been achieved in this patient," the scientists wrote last week in the journal Blood.

But there are still troubles. Other researchers warned that it was possible low-level, and hard to detect, amounts of the virus could remain in the patient. The resilient virus could also eventually evolve a way to infect patients despite the mutation. And the procedure would likely never become widely used, because the transplant therapy carries a high mortality rate.

Researchers are, however, looking at gene therapy to deliver the mutant genes to targeted cells, though that still has technical hurdles to overcome.

"All I'd say is that this is an extreme and difficult procedure and impractical on any general level -- at least right now," wrote Andrew Sullivan, an HIV-positive writer who blogs at The Atlantic. "But it does suggest that HIV can be gotten rid of entirely if you try hard enough."

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