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.