WASHINGTON - German researchers who used a bone marrow transplant to treat a cancer patient with the AIDS virus, have declared him cured of the virus - a stunning claim in a field where the word "cure" is barely whispered.

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The patient, who had both HIV infection and leukemia, received the bone marrow transplant in 2007 from a donor who had a genetic mutation known to give patients a natural immunity to the virus.

Nearly four years after the transplant, the patient is free of the virus and it does not appear to be hiding anywhere in his body, Thomas Schneider of Berlin Charite hospital and colleagues said.

"Our results strongly suggest that cure of HIV has been achieved in this patient," they wrote in the journal Blood.

AIDS researchers have rejected the approach on any kind of scale for patients with HIV. A bone marrow transplant is a last-ditch treatment for cancers such as leukemia.

It requires destruction of a patient's own bone marrow - itself a harrowing process - and then a transplant from a donor who has a near-exact blood and immune system type. Months of recovery are needed while the transplant grows and reconstitutes the patient's immune system.

"It's not practical and it can kill people," said Dr. Robert Gallo of the Institute of Human Virology at the University of Maryland, who helped discover the human immunodeficiency virus that causes AIDS.

"It is possibly a cure, that's for sure, you won't know for absolute sure until the person dies and undergoes extreme PCR (genetic) analysis of post-mortem tissue."

The mutation affects a receptor, a cellular doorway, called CCR5, that the AIDS virus uses to get into the cells it infects.

Since the 1990s scientists have known that some people, mostly of Northern European descent, have the mutation and are rarely infected with HIV.

"They are uninfectable, virtually," Gallo said.

Some researchers are working on the idea of gene therapy to treat or try to cure HIV, but the technology is still in experimental stages.

"I don't want to throw cold water on an interesting thing, but that's what it is - an interesting thing," Gallo said.

Schneider's team has been following the patient, taking samples from his colon, liver, spinal fluid and brain as he developed various conditions that justified the tests. They tested all these samples for evidence of the virus, which can be difficult to detect unless it is actively infecting cells.

All these places are suspected "reservoirs" where HIV can hide out for years, to rebound in patients who stop taking drugs that suppress the infection.

This patient appears to have a fully functioning immune system, they found, which appears genetically identical to cells from the donor - not the patient's own immune cells.

Schneider's team found no evidence of HIV anywhere.

"From these results, it is reasonable to conclude that cure of HIV infection has been achieved in this patient," they wrote.

The AIDS virus infects 33 million people globally and has killed more than 25 million since the pandemic began in the 1980s. Cocktails of strong drugs can suppress the virus, keeping patients healthy and reducing the chance they will infect others, but there is no vaccine.