Kissing is one of those behaviors that’s pretty bizarre when you think about it from an alien’s perspective: two bodies of meat wiping their faces against each other, creating suction and trading fluids. Sounds pretty gross. But recent research suggests there may be a biological advantage to swapping spit.

Oral microbiota — that is, the bacteria cultures that live in your mouth — make up a fraction of your body’s total bacteria population. All told, there are some 100 trillion microorganisms teeming within the walls of your body, most of them found in your gut. When we kiss, roughly 80 million bacteria transfer between us and our partner, introducing new and sometimes beneficial bacteria into our mouth’s ecosystem.

“Interestingly, the current explanations for the function of intimate kissing in humans include an important role for the microbiota present in the oral cavity, although to our knowledge, the exact effects of intimate kissing on the oral microbiota have never been studied,” said Remco Kort, lead author of the new study from the Microbiology and Systems Biology department at Netherland’s TNO, in a statement.

Prior research has already found that we tend to seek out people with microbiota cultures that are different from our own. Think of it as your body’s way of knowing its enemy, protecting itself from foreign invaders by exposing itself to them. Kort and his team wanted to understand the extent to which partners shared their microbiota when kissing, and as it turns out, it’s quite a lot.

Twenty-one couples filled out a questionnaire asking them how often they kissed. Then the researchers had each subject consume a probiotic yogurt which contained hearty sources of so-called “good bacteria.” After the couples shared an intimate 10-second kiss, the researchers swabbed their cheeks to check bacteria levels. The couples who reported they kissed more often ended up having more similar microbiota than less-frequent kissers.

On the one hand, this is kind of endearing. We’re similar to our partners in personality, values, and also oral bacteria, apparently. It’s also scientifically useful. “Bacteria help regulate a lot of body processes,” said Dr. Alison Morris, an associate professor of medicine at the University of Pittsburgh Medical Center, to Today. (Morris was not involved with the study.) “They also help shape the immune system. And exposure to someone else’s bacteria could help your immunity. So this study should make you want to kiss more.”

With all the advertisements for antimicrobial this and antibacterial that, it may be tempting to associate bacteria with harmful effects. But the truth is, bacteria play a vital role in keeping our immune systems strong. When “bad bacteria” infects our bodies, we rely on the good bacteria culture to fight them off. And the only way we can develop good bacteria supplies is to receive them either through our diets or donations from the environment. Kissing just so happens to be one of the more enjoyable means of transmission.

There are some other not-so-pleasant ways, mind you. A study conducted in 2013 found that infants whose parents sucked their pacifiers clean developed fewer allergies than infants whose parents rinsed or boiled them. The trend isn’t new. Scientists have known for years that children’s microbial makeup depends largely on how they’re raised, sometimes even from birth. In recognizing this trend, Kort got curious whether the same environmental effects held true for adults.

“We know that we obtain microbes that stay with us — many throughout our lives — in the first three years of life, but what’s happening later on?” he told Today. “I wanted to do an experiment where I could monitor the exchange and see to what extent we are able to colonize our partners.”

“Colonizing our partners” aside, the study offers some compelling scientific evidence for puckering up. Take that, aliens. We are normal.

Source: Kort R, Caspers M, de Graaf A. Shaping the oral microbiota through intimate kissing. Microbiome. 2014.