Recently, a colleague shared a story of going to the doctor for a cold and being asked if he was in a relationship. Puzzled, he answered yes but prompted the doctor to explain why. The answer: New couples are swapping a lot more than saliva and awkward anecdotes. When two people get together, they also combine their immune systems, so he would likely get sick more often during this period of assimilation.
Los Angeles may be full of quacks, but this doctor was right on the money. A recent study in Nature of 670 people, aged 2 to 86, found that people who share an address also share an immune system with 50 percent less variation than you’d find between two strangers. This particular conclusion was drawn from noticing that in general, our immune system keeps returning to its own same resting state even after a brief illness sets it off, but that’s not the case when you move in with someone. Then, and only then, does your immune system actually change to adapt to the new person’s immune system, as if it needed full-on siege to finally concede some real ground.
The reason is something called spousal concordance, what the researcher Adrian Liston told New Scientist is basically the idea that shacking up in a relationship means you’re going to eat similarly, move similarly, imbibe similarly, and expose yourself to the same pollution, environment, germs, Netflix shows, fast food and overall lifestyle. You also share common viruses and gut bacteria. Together, you build up a “shared resistance” to your environment.
This increases the longer you’re together, and separate research has found that couples who’ve been together for decades have similar kidney function, grip and cholesterol, as well as similar difficulty in completing tasks, and similar levels of depression. And even similar levels of skin microbes, in particularly high concentration on your feet. Sexy!
It’s not just that we pick a partner with a similar immune system from the start. It’s that, because only 25 percent of our immune system is inherited and the rest is lifestyle, that means together a couple creates a shared, deeply influential experience that tracks on the cellular and microbial level, too.
One major note from the Nature study is that all the adults in the research were also parents, and while the researchers believe anyone who cohabits would experience a similar effect, having children is particularly likely to increase the similarity in the immune system because children “reduce the sterility” of the household. “It’s not particularly nice to imagine, but the easiest way to transmit gut bacteria is the faeco-oral route – and parents could both be changing a baby’s nappy,” Liston told New Scientist. (A really, really messy roommate might accomplish the same result, though.)
But that raises another related issue: Why is it that kids get sick constantly when they enter preschool, and why does this mean a renewed period of constant sickness for you and your partner? The immune system here is also at play. In short, whereas adults may get fresh bouts of illness because of too much immune system exposure during a new relationship, for children, it’s the lack of an immune system that causes excessive illness during certain phases of growth.
When children enter a group care setting for the first time, they become exposed to a “germ onslaught” that can result in as many as eight to 12 colds in the first 12 months, or a cold a month. From Today’s Parent:
Many of the typical illnesses found in daycare settings, including the common cold, stomach bugs, conjunctivitis (pink eye), and hand, foot and mouth disease, are caused by viruses. These bugs are easily spread through direct and indirect contact between toddlers in close proximity who are likely wiping their noses, sneezing and coughing, while sharing toys and food.
This is the case even when you and the daycare are diligent about hosing the place down nightly with bleach, because it doesn’t matter how clean a place is when the immune system it’s intersecting with can’t stand the heat.
That fun continues through age 6, during which time they continue to get as many as six to eight colds a year. As epidemiologist and pediatrician Aaron Milstone tells Vox of children, “They have no preexisting immunity.” That lack of immunity also explains why they take forever to get over colds as well, with severe colds, coughs and respiratory infections taking anywhere from 15 to 25 days to wrap up. (The next time you don’t believe it when a parent you work with is out again due to a sick elementary school aged child, stuff it.)
Worst of all, parents get these illnesses all over again, too, because many of them are the ever-shifting flus and respiratory infections that grownups don’t necessarily build a resistance to for life, hitting them at a time when they are profoundly sleep-deprived and therefore most susceptible to catching it.
Adding to this ouroboros of infections is the fact that children are also uncivilized little germ spreaders who haven’t quite mastered washing their hands and not rubbing those hands on everything in sight, from their butts, to up their noses, to your butt and your nose.
But let’s face it: When you move in with someone, you’re like this too. So this is all as it should be. And there is really nothing to be done (except, please, get a flu shot). So in conclusion, take your immune system hits as they come, and ride them like a gentle wave of, well, the love flu, the kind you can bounce back from within a month, which will be just in time for the next actual cold.