In 2007, the man received a bone marrow transplant to treat his leukemia. The transplant — which treats leukemia by essentially rebooting the body's immune system and creating new white blood cells —also had the benefit of wiping out the HIV infection.

Now, three and a half years later, the patient remains HIV-free, which suggests he is cured of the disease, the researchers said.

"I'm extremely excited about the result," said Jerome Zack, a researcher at the University of California, Los Angeles, who studies HIV infection and was not involved in the study. "It suggests that at least in this one individual, there's a long-term benefit to this approach."

In the transplant, the patient was given two bone marrow stem cell transplants chosen from a donor who had a natural genetic resistance to HIV. Around one to three in every hundred people have a gene mutation which gives them a natural resistance to HIV. The mutation essentially prevents the most common form of HIV from getting inside certain immune cells, called T CD4 cells, and wreaking havoc on the immune system.

Afterward, the virus appeared to stop replicating in the patient's body, and he no longer needed HIV antiretroviral medication.

The findings, first published in the New England Journal of Medicine last year, were met with excitement. But questions remained about how long the effect would last and whether different forms of the virus could still get inside immune cells.

The results, published this week in the journal Blood, show that the donor's bone marrow cells were able to rebuild the patient's supply of CD4 cells to match that of a healthy person. None of the patient’s original CD4 cells remained. Moreover, the virus was not detected in the body, and the patient showed no signs of infection, the researchers said.

The most common form of HIV relies on a receptor on the surface of a person's CD4 cells, known as the CCR5 receptor, to get inside the cells where it causes its damage. Because of the mutation, the CD4 cells from the donor lacked this receptor, preventing HIV from attacking through this route.

However, another form of the virus uses a different receptor to get in cells, known as CXCR4 receptor. Because of the virus' high mutation rate, the patient would likely have had both forms in his body when he was given the transplant, Zach said. And the donor cells would have been extremely vulnerable to HIV infection through CXCR4, Zack said, because they rapidly divide in order to repopulate the new body, a characteristic that makes them prime targets for HIV.

So exactly why the patient doesn’t have any detectable HIV in his body remains a mystery. It is possible that through the entire course of treatment, both types of HIV were eliminated from the body, Zack said.

He added: "We don’t know if it would always happen, or whether there's still some of that virus somewhere in the body that hasn't been sampled."

Ultimately, the results would need to be reproduced before researchers could know whether this was an option for treating HIV, Zack said. And, practically, finding donors would be a challenge — only one percent of Northern Europeans are known to have this particular mutation, Zack said.

Researchers are also experimenting with ways to knock out the CCR5 receptor using gene therapy, which would be another way of potentially getting the same result, Zack said.