Between the awkward morning conversations and the fear of acquiring some new Zika-like STD, it makes sense to be wary of casual sex. But there are the upsides: orgasms, human touch, the opportunity to go through someone's medicine cabinet while they're sleeping. And then there's this: Reputable science has shown that having sex bolsters your immune system.
First, it reduces stress, a known immune suppressant. To take one example, a study published in the journal Psychosomatic Medicine followed the private lives of 51 German couples. It found that the men and women had lower levels of cortisol, a stress hormone, on days that included "long durations of intimacy," a PC term that encompasses everything from holding hands to making spatzle (that's a sex thing, right?). Another study following female students found that pre-exam stress levels were lower among those reporting the greatest relationship satisfaction (of course) and sexual activity (independent of relationship status). But, to be fair, the way the study was constructed leaves it unclear whether the increase in sex reduced stress, or the naturally low stress led to more sex. But either outcome sounds like good news.
Then there's the potential antibody boost. A study from the journal Psychological Reports found that people who reported one to two weekly grind sessions had the highest levels of immunoglobulin A, an antibody that shields you from infection. The study actually calls it the "literal first line of defense against most invading pathogens." Interestingly, more sex didn't confer more benefit. People having sex three or more times per week actually had less immunoglobulin A. Researchers hypothesize that the stressof juggling such an active bone schedule, possibly with many suiters, may be to blame.
But while the bulk of research points to the strongest immune boost in coupled partners, there's something to be said about getting touchy with new flings: With each one-night stand, you're coming into contact with a new set of germs. "We have had this notion that bacteria are bad, that they cause infection and bad things, and that's true—but there are many, many more bacteria that live on us and in us that are vital for many functions, including our immune response," says Alison Morris, director of the Center for Medicine and the Microbiome at UPMC and professor at the University of Pittsburgh School of Medicine. Certain bacteria that congregate in your gut and nose are particularly good at fighting the flu, she notes. And exposure to flora from others can help build up your bug armies to fight off pathogens.
There's some conjecture to this argument, but it's not totally without merit. The notion that isolating yourself from germs can backfire on you by handicapping your immune system is what researchers call the hygiene hypothesis. To illustrate it, Harvard researchers arranged to have mice born in a sterile environment. (You can't exactly force human babies to grow up in a lab, you know.) Compared to those reared in contact with real-world germs, the mice in sanitary living communities were more likely to have allergies, or even die. Their immune systems weren't properly built up, and the evidence suggest that a similar thing would happen if you raised humans in a sterile environment. The hygiene hypothesis applies most acutely to children, but new bacteria continue to bolster your immune system "through your lifetime," says Morris.
And hey, great news: Sex is super germy. A study published in the journal Microbiome found that a simple 10-second kiss—barely even foreplay—could pass on as many as 80 million bacteria. If you decide to opt into the sex-for-health plan, just be aware that there are, in fact, some bad bugs out there. "There are obviously certain bacteria and viruses that are pathogens that you don't want to be exposed to," Morris says. Think gonorrhea, chlamydia, syphilis, HIV—things like that. So don't be careless. You have to protect yourself against STDs.
But you also have to protect yourself agains the flu. And how you do that is your business.