When Kim met Ken, they were instantly attracted to each other. The two had many things in common. But there was one major difference they had to consider: Kim was HIV negative while Ken was HIV positive.

When HIV-positive and HIV-negative individuals become a couple and decide to have sex, “It’s important to discuss together what the health concerns might be for the two of you, and to be comfortable with your decision,” says Joanna Eveland, an HIV specialist at San Francisco’s Mission Neighborhood Health Center.

To avoid worries about transmitting the virus, people living with HIV must take antiretrovirals (ARV). This can help to lower their viral load to undetectable levels and reduce the risk of HIV transmission. But an undetectable viral load doesn’t mean transmission can’t happen. Even when HIV is undetectable in someone’s blood, the virus can be found in other bodily fluids, such as semen.

Besides recommending people take meds to lower the chance of HIV transmission, doctors suggest serodiscordant couples use condoms and get screened and treated for all sexually transmitted infections; what’s more, the HIV-positive partner should undergo regular testing to check his or her viral load.