More needs to be done to persuade people to have fewer sexual partners, according to leading HIV experts.
Over 40m worldwide have HIV
They said encouraging people to have fewer partners would result in fewer HIV infections.
Writing in the British Medical Journal, they said little effort has gone in to tackling the issue in recent years.
They said the message appears to have been lost, as campaigns put the emphasis on abstaining from sex or using condoms.
The experts, who include officials from the Global Fund for Aids, the Bill and Melinda Gates Foundation and the US Agency for International Development, said efforts in some countries to address the issue had paid off.
HIV infection rates have fallen from 15% to 5% in Uganda over the past decade. The experts said a nationwide campaign encouraging people to stick with regular partners contributed to the fall.
The 'ABC' sexual health strategy
Abstinence or delay of sexual activity
Be faithful and reduce partner numbers
They said similar campaigns in Thailand, Cambodia, Ethiopia and the Dominican Republic have shown similar results.
"It seems obvious but there would be no global Aids pandemic were it not for multiple sexual partnerships," they said.
"The rate of change of sexual partners - especially concurrent partners - is a crucial determinant in the spread of sexually transmitted infections."
The experts called for an "ABC" approach to improving sexual health - advocating abstinence, being faithful and condom use.
They said this would ensure that the message that fewer sexual partners reduces the risks of HIV was not lost.
"Rather than arguing over the merits of abstinence versus condoms, it is time for the international community to unite around a balanced, evidence-based ABC approach," they said.
In an accompanying editorial, David Wilson, a member of the World Bank's global HIV/Aids programme, welcomed the report.
"Their argument that partner reduction is the potential centre-piece of a unified ABC approach is good common sense."
However, he said the "A,B,C" approach may not work everywhere, particularly in communities where women are often forced to have sex against their will.
In another article, researchers in Canada said abstaining from sex is the best way of protecting against HIV and other sexually transmitted infections.
Stephen Genuis, an associate professor at the University of Alberta, said studies have shown a link between having sex at an early age and having many sexual partners.
"Although partner reduction is a critical and often overlooked component in the ABC strategy, we believe that delayed sexual debut for young people should be the first step in programmes to prevent sexually transmitted infections."
The UK charity Terrence Higgins Trust expressed doubts about the ABC strategy.
"I take the idea of promoting monogamy with a large pinch of salt," said Will Nutland, its head of gay men's health promotion.
"What has worked in Uganda doesn't necessarily translate to London.
"In some societies would people really be able to implement this, where women don't necessarily have control over their sexual encounters."