A London man appears to be free of the AIDS virus after a stem cell transplant, the second success for this rare and difficult treatment plan, doctors reported. Such transplants are dangerous and have failed in other patients. They're also impractical to try to cure the millions of people worldwide who are infected with HIV.

The therapy had an early success with Timothy Ray Brown, a U.S. man treated in Germany and originally known as the "Berlin patient," who is 12 years post-transplant and still free of HIV. Until now, Brown has been the only person thought to have been cured of infection with HIV, the virus that causes AIDS.

The latest case "shows the cure of Timothy Brown was not a fluke and can be recreated," said Dr. Keith Jerome of Fred Hutchinson Cancer Research Center in Seattle, who had no role in the case. He added that it could lead to a simpler approach that could be used more widely.

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The case was published online Monday by the journal Nature and is being presented at an HIV conference in Seattle.

The London patient has not been identified. He was diagnosed with HIV in 2003 and started taking drugs to control the infection in 2012. It's unclear why he waited that long. He developed Hodgkin lymphoma that year and agreed to a stem cell transplant to treat the cancer in 2016.

With the right kind of donor, his doctors figured, the London patient might get a bonus beyond treating his cancer: a possible HIV cure.

Doctors found a donor with a gene mutation that confers natural resistance to HIV. About 1 percent of people descended from northern Europeans have inherited the mutation from both parents and are immune to most HIV. The donor had this double copy of the mutation.

That was "an improbable event," said lead researcher Ravindra Gupta of University College London. "That's why this has not been observed more frequently."

The transplant changed the London patient's immune system, giving him the donor's mutation and HIV resistance. The patient voluntarily stopped taking HIV drugs to see if the virus would come back.

Usually, HIV patients expect to stay on daily pills for life to suppress the virus. When drugs are stopped, the virus roars back, usually in two to three weeks.

That didn't happen with the London patient. There is still no trace of the virus after 18 months off the drugs.

Stem cell transplants typically are harsh procedures which start with radiation or chemotherapy to decimate the body's existing immune system and make room for a new one. There are frequently complications, too. Brown had to have a second stem cell transplant when his leukemia returned.

For these reasons, some experts are cautioning not to put too much weight on this case.

"This approach is risky, not feasible, and not scalable, and so while interesting it really does not advance the field very much," Dr. Anthony Fauci, director of the National Institute of Allergy & Infectious Diseases, told CBS News chief medical correspondent Dr. Jon LaPook.

However, Dr. Rowena Johnston, vice president and director of research at the AIDS research organization AMFAR, is more optimistic and says it means "that the research is going in the right direction."

If the London patient ends up being the second confirmed cured case, "then what we can do is to start to put these lessons together that we're learning from each of these patients and more patients that we have through AMFAR's research patients in the pipeline we can put that information together and really learn how to build a cure for HIV that we can then deliver to the whole world," she said.

Compared to Brown, the London patient had a less punishing form of chemotherapy to get ready for the transplant, didn't have radiation and had only a mild reaction to the transplant.

Dr. Gero Hutter, the German doctor who treated Brown, called the new case "great news" and "one piece in the HIV cure puzzle."

Brown said he would like to meet the London patient and would encourage him to go public because "it's been very useful for science and for giving hope to HIV-positive people, to people living with HIV," he told The Associated Press Monday.