For the same reasons you wouldn’t share a fork with a stranger or, say, lick an ATM, inserting your tongue into someone else’s mouth can set your body up for disaster. Here’s how:
1. It can expose you to nasty bacteria that make you feel miserable.
“Mouths can serve as a transmission route for germs because there is close connection with the gastrointestinal and respiratory systems, and these are common sites of infections for germs,” explains Kelly Reynolds, PhD, a microbiologist at the University of Arizona’s Zuckerman College of Public Health. And that goes for people who seem perfectly healthy, since humans can shed pathogens through saliva for a few days before and after experiencing symptoms.
As you might imagine, open-mouthed kissing is particularly icky: It transfers more germs than closed-mouth kissing, and the more germs you’re exposed to, the more likely you are to get sick, according to Reynolds.
While not kissing anyone at all is the best protection, simply not kissing people who have a fever (a telltale sign they’re contagious), seem run down, or feel like they’re getting sick can also help. Otherwise, kissing could mean you get a strep or staph bacterial infection, or in some cases, a cold or the flu (although you’re actually more likely to contract those from inhaling the particles an infected person expels when they cough, since germs suspended in the air are more likely to be inhaled deep into the respiratory passage, according to Reynolds).
2. It can spread viruses like mononucleosis and meningitis that can put you out for weeks, if not months.
They’re two of the most commonly transmitted pathogens via kissing, according to Reynolds. Since the mono virus can persist and cause symptoms like extreme fatigue for upward of six months in some people, and meningitis, the inflammation of the membranes surrounding your brain and spinal cord, can last up to 10 days, sometimes causing permanent and even life-threatening damage, neither conditions are remotely desirable.
3. It can lead to a sexually transmitted infection.
As you (hopefully) learned in sex ed, microbes that cause gonorrhea, syphilis, herpes, and HPV can be transmitted through saliva. Often asymptomatic, these can easily be passed between partners in acts as innocent as kissing.
4. It can lead to cold sores.
Kissing can spread cold sores, an infection caused by the herpes virus that’s marked by fluid-filled blisters around the mouth. Since the virus can be contagious regardless of whether sores are visible, kissing someone who doesn’t appear to be affected can still lead to a mouth infection.
5. It can expose you to blood-borne viruses like HIV.
While HIV is typically transmitted through semen, vaginal fluids, and blood, kissing can lead to transmission on the off-chance that both you and an infected partner happen to have tears in your gums, which can be caused by brushing too hard, aggressive flossing, or early stages of gingivitis, all of which create an opportunity for blood to be exchanged, according to Reynolds.
6. It can cause cavities.
Because kissing can expose you to bacteria found in your partner’s dental plaque and cavities, according to Reynolds, cavities are considered contagious. And that’s regardless of whether a partner’s breath seems minty fresh, since anyone you kiss can pass an oral infection.
7. It can trigger a serious allergic reaction.
Some 12 percent of people who kiss someone who’s eaten a food they’re allergic to will experience an allergic reaction, according to a Swedish survey on kissing with food allergies. You’re more likely to experience a reaction when your partner’s just eaten a food you can’t tolerate — but your sensitivity depends on the severity of your allergy, as do your symptoms, which can include hives, facial swelling, shortness of breath, and vomiting that can occur within minutes of exposure. It’s why it’s smart to make sure your partner is aware of your allergies before locking lips.