Nairobi - HIV is being spread by "wife swapping" and infidelity according to a report in Kenya, where most new infections occur among heterosexual couples in stable relationships.
In Nairobi, couples have adopted "MBA" - Married But Available - behaviour, where they openly have other sexual partners, Kenya's Daily Nation newspaper said.
"It is a worrying trend that is gaining momentum," Alloys Orago, director of Kenya's National Aids Control Council, is quoted as saying in the report.
"There is a puzzling attitude, which is growing among young people, who go on outings in groups and later on engage in wife swapping," he said.
People in a steady heterosexual relationship make up 44 percent of new HIV infections.
This is nearly triple the rate of new infections among men who have sex with men (15 percent) and sex workers (14 percent).
Women make up almost six out of 10 of adults living with HIV in Kenya. Transmission to newborns is also high, with nearly 13,000 babies infected during birth in 2011, according to government statistics.
Experts say that it is often hard for married women to insist on condom use because of the threat of violence and their economic dependence on men.
Nairobi has the highest number of people in Kenya living with HIV/AIDS, at 199,100 people, and also the highest number of new HIV infections in 2011, at 13,510 people, according to the report.
Its prevalence rate - the rate of infection among those over 15 years old - is 8.6 percent, higher than the national average of 6.2 percent, the Daily Nation said.
Around 1.6 million Kenyans live with the disease.