It must be stated that PrEP is only effective against HIV and not other sexually transmitted infections such as herpes or chlamydia to name a few. So if either partner is having sex outside the relationship, whether you're negative or positive, you're both at risk to possible exposure to STIs. When a person living with HIV begins an antiretroviral treatment regimen, their effectively no risk of sexually transmitting the virus to an HIV-negative partner. Couples with one person who is HIV-positive and one who is HIV-negative are HIV isn't the first topic that comes up when most couples start dating. You may. The thing is, he's HIV positive and undetectable. case you didn't know, PrEP is a daily pill that's highly effective at keeping you HIV negative.