The findings show that treating those with HIV can not only help them live longer with the often fatal and incurable disease but can also be a powerful way of limiting the virus' spread.

Researchers found that since the introduction of a treatment plan called highly active antiretroviral therapy (HAART) for HIV patients in the Canadian province of British Columbia in 1996, the number of new HIV diagnoses has fallen by 52 percent.

Their study also found that rates of other sexually transmitted diseases went up, suggesting that it was the AIDS drugs and not other confounding factors such as condom use or less sexual activity, that produced a fall in HIV infections.

The results show that for every 100 patients placed on HAART new HIV diagnoses fell by 3 percent, suggesting that this type of treatment could significantly reduce the spread of the human immunodeficiency virus (HIV) that causes AIDS.

Experts commenting on the findings, which were reported at an international AIDS conference in Vienna on Sunday and in the Lancet journal, said they should be used to shape future treatment plans.

"Experiences such as those reported today should be strongly considered by clinicians, national and international agencies, (and) policymakers," said Franco Maggiolo and Sebastiano Leone of the Division of Infectious Diseases, Ospedali Riuniti in Italy, in a commentary in the Lancet. "HAART might play an important part in the future control of the HIV epidemic."

The AIDS virus infects 33.4 million people around the world and has killed 25 million since the pandemic began in the 1980s. There is no cure and no vaccine but drugs can keep patients healthy. Without treatment, the virus destroys the immune system, leaving patients susceptible to infections and cancer.

More than 20 drugs are now on the market and can be combined in various ways to control HIV, although it usually mutates eventually and patients must switch to different combinations to keep it under control. Drugmakers include Gilead, GlaxoSmithKline, Pfizer, Merck & Co, Bristol-Myers Squibb and Abbott Laboratories.

The Canadian team, led by Julio Montaner at the British Columbia Center for Excellence in Vancouver, analyzed data on HAART and looked at viral load of HIV patients -- the level of virus in their bodies -- and at new HIV diagnoses in the province, where residents get free HIV care.

HAART involves treatment with three or more AIDS drugs, which can be either more expensive branded medicines or cheaper generics, which are available at knocked-down prices in poorer countries.

During three distinct time periods, researchers saw that the number of people receiving HAART had a strong impact on viral load and new diagnoses. As HAART coverage increased sharply, new HIV diagnoses decreased sharply, and as HAART coverage stabilized, so did viral load and new HIV diagnoses.

"Our results show a strong and significant association between increased HAART coverage, reduced community viral load, and decreased number of new HIV diagnoses," Montaner said.

The annual number of new HIV infections around the world was 2.7 million in 2008, the same as in the previous year. This was down from 3 million in 2001.

(Editing by Alison Williams)