Drugs used to suppress the HIV virus could also be effective at preventing people from contracting the disease, a study has found.
Researchers set out to find out whether people with HIV who use antiretroviral therapy (ART) are less likely to infect their partners who do not have the disease.
They found that ART prevented partners passing on the disease in 92% of cases.
The drug was given to 349 people who had HIV. A total of 3,400 people were included in the study, made up of couples where only one person already had HIV. Of the 103 people that contracted the disease during the study, only one was the partner of someone who had been given ART drugs.
This has led the researchers to conclude that the drug could be used to prevent as well as control the virus.
Dr Deborah Donnell of the Vaccine and Infectious Disease Institute at the Fred Hutchinson Cancer Research Center in Seattle, said: "These observational data strongly support the hypothesis that ART substantially reduces HIV infectiousness and transmission risk."
Dr Donnell said this was because the drugs make HIV less concentrated in the blood, so people are less infectious. Research is now being conducted to see if the effects of ART are lasting.
The results of the study have been published in The Lancet.