Social contact can clearly spread disease: That’s why we lean away from snotty hugs, tell sick colleagues to go home, and quarantine people during epidemics. But the germs behind infectious illnesses are but a tiny fraction of our full microbiome—the microbes that share our bodies. Most of these are harmless, perhaps even helpful. And they can hop between individuals, too.
A growing number of studies, including two recent ones with chimps and baboons, have shown that social interactions affect the composition of the microbiome. Through hugs, handshakes, and even hip-checks, we translate our social networks into microbial ones, transferring benign or beneficial microbes to our neighbors, and acquiring theirs in return.
This means that there’s a “pan-microbiome”—a meta-community of microbe species that spans a group of hosts. If you compare your microbiome to your private music collection, the pan-microbiome is like the full iTunes store, and every handshake is an act of file-sharing.
There’s some evidence that humans share microbes through physical contact. In one study, people who share living quarters end up with similar microbes. In another, the skin microbes of opposing roller-derby teams converge during a game. But these were snapshots. To study how social ties affect the microbiome, you’d ideally want to track people over long periods—everything from the friends they hung out with to the bacteria in their poop. “You’d have to invade their privacy to an extent that most people probably wouldn’t put up with,” says Andrew Moeller from the University of California, Berkeley.