As we sink into the prime season for colds and flus, it’s a good time to remember that most kinds of viruses that invade our bodies aren’t interested in us. They attack the bacteria inside us instead.

There are probably 100 trillion microbes inside each of us, such that our bodies are ten microbes for every one human cell. Those tenants probably belong to several thousand species, with a collection of genes that’s perhaps a 100 times bigger than the human genome. These microbes live in our guts, lungs, mouths, noses, skin, and many other nooks and crannies. Far from making us sick, they help us in many ways, making food for us, defending us from invaders, and nurturing our