The ability of the HIV genome, or reproductive code, to hide out in cells and be revived after decades poses a major obstacle in the quest for a cure.

Being able to expose the virus in its hiding place would allow scientists to target the host white blood cells in a killing blitz.

"It is the beginning of work toward a cure for AIDS," David Margolis, co-author of the study published in the journal Nature, told AFP as the International AIDS Conference was under way in Washington.

HIV is a retrovirus, inserting its DNA into the genome of host white blood cells, CD4+T cells in this case, and turning them into virus factories. Sometimes it goes into hiding in some cells even as others keep on producing.

Some 34 million people around the world are living with HIV, which destroys the immune system and has caused about 30 million AIDS-related deaths since the disease first emerged in the early 1980s.

In the latest study, researchers in the United States used the chemotherapy drug vorinostat to revive and so unmask latent HIV in the CD4+T cells of eight trial patients.

The patients were also on antiretroviral drugs, which stops HIV from multiplying but have to be taken for life because they do not kill the virus hidden away in reservoirs.

"After a single dose of the drug, at least for a moment in time, (vorinostat) is flushing the virus out of hiding," Margolis said of the trial results -- the first drug ever shown to do so.

"This is proof of the concept, of the idea that the virus can be specifically targeted in a patient by a drug, and essentially opens up the way for this class of drugs to be studied for use in this way."

The drug targets an enzyme that allows the virus to lie latent.

The researchers cautioned that vorinostat may have some toxic effects and stressed this was merely an early indication of feasibility that had to be explored further.

Exactly what would happen after the virus was unveiled in reservoir cells was also not certain, said Margolis.

"We know that many cells that produce HIV die in the process. We know many cells that produce HIV can be identified and killed by the immune system. As far as we can tell, all the viruses floating around while patients are taking therapy don't get into cells because they are blocked by the therapy," he said.

Without a host cell, the virus would die within a few minutes.

"There is a possibility that this could work. But ... if it is only 99 percent true and one percent of the virus escapes, it won't succeed. That is why we have to be careful about our work and what we claim about it."

In a comment published with the study, HIV researcher Steven Deeks said the research provided "the first evidence that ... a cure might one day be feasible".

But, as is common with early clinical trials, the study raised more questions than answers -- including ethical concerns about giving potentially toxic drugs to HIV-infected people who are otherwise healthy, he said.

"These data from the lab of David Margolis are genuinely exciting for those exploring pathways to achieving a cure for AIDS," Oxford University HIV researcher John Frater told AFP, calling for investment in further research.

HIV immunologist Quentin Sattentau called the findings promising, but said other types of reservoir cells, including in the brain, may not respond to this treatment.

"Thus there is a long way to go before we will know if this can work to completely eradicate HIV from an infected person."

Timothy Ray Brown, 47, an American from Seattle, Washington, rose to fame as the so-called "Berlin patient" after doctors tried a novel technique to use an HIV-resistant donor for a stem cell transplant to treat Brown's leukemia.

Since 2007, he has had two high-risk bone marrow transplants and continues to test negative for HIV, stunning researchers and offering new pathways for research into how gene therapy may lead to a more widely acceptable approach.

"I am living proof that there could be a cure for AIDS," Brown told AFP in an interview. "It's very wonderful, being cured of HIV."

Brown looked frail as he spoke to reporters in Washington where the 19th International AIDS Conference, the world's largest meeting of scientific experts, policymakers and advocates is taking place.

The bone marrow transplant he received carried significant risks and may be fatal to one in five patients who undergo it. But he said his only complaint these days is the occasional headache.

He also said he was aware that his condition has generated some controversy, but disputed the claims of some scientists who believe he may still have traces of HIV in his body and may remain infectious to others.

"Yes, I am cured," he said. "I am HIV negative."

Brown said he fully supports more aggressive efforts toward finding a universal cure, and has met with a number of top scientists in recent days who have treated him "like a rock star."

He said he hopes to harness some of that fame to encourage donors to fund more research, and noted that Europe and China spend far more on cure research than the United States.

"There are thousands of very able researchers who cannot get funded for research, so I want to change that. And there are a lot of researchers who are willing to work to find a cure for HIV."

Brown was a student in Berlin, Germany, when he tested positive for HIV in 1995 and was told he probably had about two years to live.

But combination antiretroviral therapy emerged on the global market a year later, and eventually transformed HIV from a death sentence into a manageable condition for millions of people worldwide.

Brown tolerated the medications well but due to persistent fatigue he visited a doctor in 2006 and was diagnosed with leukemia. He underwent chemotherapy, which led to pneumonia and sepsis, nearly killing him.

His doctor, Gero Huetter, had the idea of trying a bone marrow transplant using a donor who had a CCR5 receptor mutation.

People without that receptor appear to be resistant to HIV because they lack the gateway through which the virus can enter the cells. But such people are rare, and are believed to consist of one percent of the northern European population.

It would be an attempt to cure cancer and HIV at the same time.

Brown's leukemia returned in 2007, and he underwent a bone marrow transplant using stem cells from a CCR5 mutation donor, whom he has never met in person. He stopped taking antiretrovirals at the same time.

He soon had no HIV detectable in his system. His leukemia returned though, and he underwent a second bone marrow transplant in 2008, using stem cells from the same donor.

Brown said his recovery from the second operation was more complicated and left him with some neurological problems, but he continues to be free of leukemia and HIV.

Asked if he feels like his cure was a miracle, Brown was hesitant to answer.

"It's hard to say. It depends on your religious belief, if you want to believe it's just medical science or it was a divine intervention," he told AFP. "I would say it's a little bit of both."