By Richard Black
BBC science correspondent
Medical research from Taiwan has found that giving Aids drugs to people infected with HIV can reduce transmission of the virus.
People taking Aids drugs are less likely to pass on HIV
Researchers say it is strong evidence that providing Aids drugs to infected people can be an effective way of curbing the epidemic.
In 1997, Taiwan began providing modern, highly effective anti-retroviral drugs to HIV-positive people free of charge.
The researchers say that since then, the infection rate has halved.
Meanwhile, the rates for other sexually transmitted diseases - syphilis and gonorrhoea - have remained constant.
This proves, the researchers say, that it really is the drugs, rather than any changes in people's behaviour, which are making the difference.
Study after study has shown that modern anti-retroviral drugs keep HIV-positive people alive, by suppressing levels of the virus in their bodies.
But whether this also reduces the likelihood of them passing the virus on to other people has been a matter of intense debate.
This new study, led by scientists at National Taiwan University and reported in the Journal of Infectious Diseases, provides the strongest evidence yet.
Independent experts say this study provides another reason why anti-retrovirals should be introduced into developing countries as soon as possible.
But they emphasise that other control measures, such as education and free condoms, are also needed if the current global infection rate of around five million people per year is to be reduced.
Keith Alcorn, senior editor of NAM, a UK-based HIV information service, said: "This study shows that in a country with a relatively small epidemic, introducing early treatment for all who need it could curb the spread of HIV.
"It remains to be seen if it will have the same effect in countries like South Africa, where one in five are infected and treatment will begin quite late in the course of the disease.
"The key influences will be making sure that treatment is rolled out quickly, and providing all the support that people need to take their medicines every day."
Jo Robinson, at the UK HIV charity Terrence Higgins Trust, said: "This research shows that HIV treatment can play an important part in reducing HIV transmission, but it's only part of the story.
"People with HIV should have access to HIV treatment as an important right for their own health, but we also need more choices in HIV prevention, including more investment into HIV vaccines and microbicides.
"It is also vitally important that we provide good quality health promotion information to people who are
most at risk from HIV, as well as making condoms readily available."