Sero-discordant is a term commonly used to describe a couple in which one partner is HIV positive and the other is HIV negative .
Scott Billy, director of Society for Family Health, said whether a couple remains sero-discordant depends on how often they have sex, what kind of sex, whether they practise safe sex and whether the HIV-positive partner is on treatment.
During the first month or so after someone is infected with HIV, they are hyper-infectious and are more likely to infect others because they have more of the virus in their body.
"If a couple did not have sex during this hyper-infectious period then they are more likely to be sero-discordant," Billy said.
There is no database of sero-discordant couples in South Africa but Billy said globally about 10% of couples who get tested together were sero-discordant.
He said several studies were under way to unravel this health riddle that is posing a grave challenge on prevention, treatment and awareness drives.
"Anything that a sero-discordant couple can do to reduce the chances of HIV transmission, such as using condoms, makes it far more likely that the couple will remain sero-discordant."
Billy said it was crucial to explain that sero-discordant couples were common and that it was important for couples to get tested together.
In April, the World Health Organisation released discordant couples guidelines after a study conducted in nine countries, including South Africa, found that chances of the infected partner infecting the uninfected partner decreased by 96% if the infected partner is on ARV therapy.
A second study found that treating the uninfected partner with ARVs, also known as Pre-Exposure Prophylaxis, reduced the risk of transmission by 73%.
The World Health Organisation recommends antiretroviral therapy for infected partners even if they do not require it for their own health to reduce the risks of infecting their uninfected partners.
"When a couple uses a combination of several strategies - such as condom use, circumcision and treatment - it's very likely that the HIV-negative partner can remain negative," Billy said.
He was critical of Bethuel Moloto and Sonto Mabaso's unprotected sex regime because Moloto was putting himself at risk.
"It's likely that this man will become HIV-positive if they continue. The best strategy is to start using condoms," he said.