LONDON- An HIV-positive man in Britain has become the second known adult worldwide to be cleared of the AIDS virus after he received a bone marrow transplant from an HIV resistant donor, his doctors said.

Almost three years after receiving bone marrow stem cells from a donor with a rare genetic mutation that resists HIV infection — and more than 18 months after coming off antiretroviral drugs — highly sensitive tests still show no trace of the man's previous HIV infection.

"There is no virus there that we can measure. We can't detect anything," said Ravindra Gupta, a professor and HIV biologist who co-led a team of doctors treating the man.

The case is a proof of the concept that scientists will one day be able to end AIDS, the doctors said, but does not mean a cure for HIV has been found.

Gupta described his patient as "functionally cured" and "in remission", but cautioned: "It's too early to say he's cured."

The man is being called "the London patient", in part because his case is similar to the first known case of a functional cure of HIV - in an American man, Timothy Brown, who became known as the Berlin patient when he underwent similar treatment in Germany in 2007 which also cleared his HIV.

Brown, who had been living in Berlin, has since moved to the United States and, according to HIV experts, is still HIV-free.

Timothy Ray Brown, also known as the Berlin patient, was the first person to be cured of HIV infection, more than a decade ago. Manuel Valdes / AP

Some 37 million people worldwide are currently infected with HIV and the AIDS pandemic has killed around 35 million people worldwide since it began in the 1980s. Scientific research into the complex virus has in recent years led to the development of drug combinations that can keep it at bay in most patients.

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Gupta, now at Cambridge University, treated the London patient when he was working at University College London. The man had contracted HIV in 2003, Gupta said, and in 2012 was also diagnosed with a type of blood cancer called Hodgkin's Lymphoma.

In 2016, when he was very sick with cancer, doctors decided to seek a transplant match for him. "This was really his last chance of survival," Gupta told Reuters in an interview.

The donor — who was unrelated — had a genetic mutation known as 'CCR5 delta 32', which confers resistance to HIV.

The transplant went relatively smoothly, Gupta said, but there were some side effects, including the patient suffering a period of "graft-versus-host" disease - a condition in which donor immune cells attack the recipient's immune cells.

Most experts say it is inconceivable such treatments could be a way of curing all patients. The procedure is expensive, complex and risky. To do this in others, exact match donors would have to be found in the tiny proportion of people — most of them of northern European descent — who have the CCR5 mutation that makes them resistant to the virus.

Specialists said it is also not yet clear whether the CCR5 resistance is the only key — or whether the graft versus host disease may have been just as important. Both the Berlin and London patients had this complication, which may have played a role in the loss of HIV-infected cells, Gupta said.

In an interview with NBC News, Dr. Anthony Fauci said the finding was not unexpected, given that the similar type of transplant was done before with Brown, but that it is not a "scalable, feasible" way to treat HIV.

"This is something that was done under the very special circumstance of an individual who had Hodgkin’s disease and needed a bone marrow transplant for his underlying cancer-like disease," said Fauci, director of the National Institute for Allergy and Infectious Diseases, part of the National Institutes of Health. "The misperception that’s floating out there now is that bone marrow transplantation is now a cure for HIV infection and that is completely untrue."

In Fauci's view, the priority should be getting patients access to the "extremely efficacious medications that we have."

The treatment called pre-exposure prophylaxis, or PrEP, is more than 99 percent effective in preventing infection if patients take the medication every day.

"In the overwhelming majority of people, you could bring down the virus to below detectable levels that would save the life of the individual, as well as make it impossible for that person to spread the virus to someone else," Fauci said.

While the bone marrow transplant finding is very promising, Dr. Daniel R. Kuritzkes said it's not the kind of therapy that could be used on healthy patients with HIV.

“It’s highly unlikely that stem cell therapy will be the way to go,” said Dr. Daniel R. Kuritzkes, director of AIDS research at Brigham and Women's Hospital in Boston who has treated HIV patients in the U.S. with bone marrow transplants, told NBC News. “There are too many risks. There have been five or six others treated in a similar way who died either from their underlying cancer or from complications of the treatment."

Although the procedure is not a new concept, it does show researchers a way forward, said AIDS expert Jonathan Jacobs, founder of the Center for Special Studies at NewYork-Presbyterian/Weill Cornell Medical Center. “So there is promise here, but the practical reality is that this treatment can be fatal and it’s very expensive."

Gupta said his team plans to use the findings to explore potential new HIV treatment strategies. "We need to understand if we could knock out this (CCR5) receptor in people with HIV, which may be possible with gene therapy," he said.

The London patient, whose case was reported in the journal Nature and presented at a medical conference in Seattle Tuesday, has asked his medical team not to reveal his name, age, nationality or other details.

With additional reporting from NBC News contributor Linda Carroll