Two men from Boston who were already supposedly cured from the dreaded HIV AIDS virus after undergoing bone marrow transplants have been infected again by the same virus.

Scientists and doctors working on their case admitted the resurgence of the fatal virus was a mystery and disappointment, if it were to be compared with the case of Timothy Brown, patient with leukemia and who is believed to be the first person ever to be cured of HIV AIDS virus after undergoing a bone marrow transplant. Also known as the Berlin patient, Mr Brown has shown no sign of the virus relapse for six years.

Gananma, a three-year-old who lost her parents to AIDS two years ago and is herself infected by the virus, plays with a volunteer at a HIV/AIDS hospice, founded by a member of the National League for Democracy (NLD) party, in the suburbs of Yangon, December 1, 2013. World AIDS day is observed on December 1, 2013. REUTERS/Soe Zeya Tun

"The return of detectible levels of HIV in our patients is disappointing, but scientifically significant," Timothy Heinrich, a physician-researcher in the Division of Infectious Diseases at Brigham and Women's Hospital in Boston.

The two HIV-positive men received their respective bone marrow transplants in 2008 and 2010. The procedure was recommended to treat their Hodgkin's lymphoma.

Eight months after their medical procedures, both men tested HIV negative, enabling the patients to stop taking antiretroviral drugs.

However, the HIV virus proved to be persistent. Traces of the virus were found in the blood of the first patient 12 weeks after stopping therapy. The second patient had his after 32 weeks.

Although HIV can be reduced to undetectable levels by very sensitive research assays, Mr Heinrich admitted "we have discovered the HIV reservoir is deeper and more persistent than previously known. Our current standards of probing for HIV may not be sufficient to inform us if long-term HIV remission is possible if antiretroviral therapy is stopped."

Dr Daniel K. Kuritzkes, another researcher at Brigham and Women's Hospital, said the cases of the two men show the HIV virus may continue to exist regardless of its absence in the blood.

"We need to develop better and more sensitive tools to detect the virus as we continue to pursue novel strategies for HIV eradication," he said. "Our results also show that the immune system can play a major role in reducing the viral reservoir, but may not be able to do the job alone. It is likely that a combination of drugs and immune therapies that target the reservoir will be needed to establish long-term remission of HIV infection."