Francoise Barre-Sinoussi, who won the Nobel Prize in Medicine in 2008 as part of a team that discovered the human immunodeficiency virus (HIV), which causes AIDS, said scientific research was zeroing in on a cure for the illness.

She cited a patient in Berlin who appears to have been cured through a bone marrow transplant, "which proves that finding a way of eliminating the virus from the body is something that is realistic."

Other sources of optimism are the small minority of patients -- less than 0.3 percent -- who exhibit no symptoms of the virus without ever receiving treatment; and a small group in France who received antiretroviral drugs and now live without treatment or symptoms, Barre-Sinoussi said.

"There is hope... but don't ask me for a date because we do not know."

She also said that it would be possible "in principle" to eliminate the AIDS pandemic by 2050, if barriers to drug access could be eliminated.

The main barriers there were not scientific but political, economic and social, she said: the problem was lack of access to testing and drugs in poor and rural areas, as well as the stigma around the virus, which undermines early detection and treatment.

Some 25,000 people -- including celebrities, scientists and HIV sufferers -- are expected in the US capital on Sunday to call for more strident global action to address the three-decade AIDS epidemic.

Deaths and infections are down in the parts of the world most ravaged by the disease, while the number of people on treatment has risen 20 percent from 2010 to 2011, reaching eight million people in needy countries.

However this is only about half the people who should be on treatment worldwide, suggesting much more remains to be done.

More than 34 million people worldwide are living with HIV, a higher number than ever before, and around 30 million have died from AIDS-related causes since the disease first emerged in the 1980s, according to UNAIDS.