I hate to break it to you, but when it comes to passing STIs to other partners, virtually anything is possible. But it doesn’t mean it’s likely.
The truth is STIs can be transmitted through kissing, but hardly because of saliva. When anyone has an open sore or cut in their mouth (sometimes we can even get them from brushing our teeth too hard), it makes it more likely to exchange blood — and blood is the major culprit in STI transmission.
While getting an STI through kissing is possible, it's very rare so no need to worry so much. If you think about it, preventing STI transmission is fairly easy so long as you’re aware of how it's spread.
Here are some to keep in mind:
By far, herpes is the most common STI that can be contracted through kissing. It’s a viral infection that is transmitted from either skin-to-skin contact or due to an exposure of mucosal tissues to the infection. A person doesn’t need to have a breakout necessarily for it to transmit either, and kissing someone who does have a breakout makes the chances even worse.
While herpes is relatively easy to treat, it can be quite painful to go through. Often times, oral herpes can result in irritating blisters or cold sores on the lips and mouth. So before you lean in for a goodnight kiss, you might want to inspect the lip region just to be sure — or even better, ask your partner.
Hep B can be passed through saliva and open sores, though probably its highest form of transmission is through anus-to-mouth sex play, seeing as hep B very often lives in fecal matter. That being said, if you and your partner kiss after anal play (and one of you has hep B and doesn’t realize it), you might be in for some trouble. If left untreated, hep B can cause serious damage to the liver.
Another tricky thing about hep B is because it often exists in fecal matter, the risk of contamination is everywhere — from toothbrushes, razors, even glasses and plates — unless you make sure you’re washing your hands after the bathroom. This is also why hep B contamination is rampant in daycares.
In order for syphilis to exchange from one person to another during kissing, there needs to be an open sore or cut inside the mouth. The same can be said for all other STIs where the requirement of blood or bodily fluids is mandatory. Otherwise, kissing is relatively safe.