The HSV2 virus causes genital herpes
A common treatment for herpes can delay the need for HIV drugs in people with both infections, say US researchers.
A study of 3,300 patients in Africa found aciclovir reduced the risk of HIV progression by 16%, The Lancet reports.
Although a "modest" effect, the researchers said the cheap treatment was a simple way of keeping people with HIV healthy for longer.
One expert said it was important to note that aciclovir did not seem to make HIV patients less infectious.
The researchers from the University of Washington, Seattle, concentrated on people infected with HIV-1 - the most common type of infection.
It is known that most people who are infected with HIV-1 are also infected with herpes simplex virus type 2 (HSV2), or genital herpes.
Previous studies have shown that keeping the herpes virus suppressed reduces HIV levels but it was unclear whether this would slow down the disease.
Those in the trial were either given a twice daily dose of aciclovir or a dummy pill and then they were monitored for two years.
At the end of the study, 284 people on aciclovir had either started taking HIV medication, had a drop in CD4 count suggesting they should be on medication or had died. The comparable figure for patients taking the placebo was 324.
Use of aciclovir treatment did not reduce HIV transmission to their heterosexual partners.
The researchers pointed out that HIV treatment with antiretroviral drugs would probably have a greater effect on reducing HIV disease progression than was seen with aciclovir.
But the herpes treatment may provide an additional option for individuals who have not reached medical thresholds for initiating antiretroviral therapy.
"Further investigation is needed to establish if suppression of this herpes virus has a role in HIV-1 treatment for people not eligible for antiretroviral therapy."
Study leader, Dr Jairam Lingappa, said: "While the HIV disease ameliorating effect we have observed is modest, it could add one more tool to help people with HIV infection stay healthy for longer."
Gus Cairns, editor of HIV Treatment Update, said: "It's nice to see a positive result in this field.
"There are biological reasons to believe that treating people's herpes could make them less likely to acquire HIV, or less likely to transmit it if they already have it, but results of trials testing the idea have been disappointing.
"Now at least we find that aciclovir, a very cheap, non-toxic and widely-available drug, can prolong the time some patients may be able to stay off the more expensive, and sometimes toxic, HIV drugs."
He added that the delay in HIV progression seen in the study may translate into a year or two off HIV medications.
"The only reservation I have is that aciclovir doesn't appear to make people less infectious, whereas the HIV drugs certainly do."