A worldwide study into HIV transmission has found men and women diagnosed with the virus can dramatically reduce the risk of infecting a partner if the carrier starts treatment while still relatively healthy.
Scientists found infection rates could be reduced by up to 96% if treatment with anti-retroviral drugs was started quickly.
The United States National Institutes of Health sampled 1763 couples in which one partner was infected by HIV.
It was abandoned four years early as the trial was so successful.
The World Health Organization said it was a ''crucial development''.
The BBC's health correspondent says the study began in 2005 at 13 sites across across Africa, Asia and the Americas.
HIV-positive patients were split into two groups. In one, individuals were immediately given a course of anti-retroviral drugs.
The other group only received the treatment when their white blood cell count fell.
Both were given counselling on safe sex practices, free condoms and treatment for sexually transmitted infections.
There was only one case of transmission between partners among those immediately starting anti-retroviral therapy. In the other group there were 27 HIV transmissions.
The World Health Organisation says sexual transmission accounts for 80% of all new HIV infections.
The BBC correspondent says the value of anti-retrovirals, in preventing transmission, had been speculated for some time after observational studies, but researchers say this is the first time it has been proven in clinical trials.