Let us make humankind in our image,” said the triune God. And then he made us plural, too. “Male and female he created them,” but we are even more plural than that phrase indicates. Each of us is plural.

We might picture our “self” as a single body. We know we’re a grand collection of cells, trillions of microscopic units that do everything from moving blood to processing nutrients into energy. But when we think about these cells, we take comfort that together they’re all one “me,” a huge organism sharing one DNA code that all started from one fertilized egg cell.

True, we are that. But we are more: Each of us is a collection of communities, millions of millions of organisms working together, with very different DNA. We have about as many bacteria and other microbes in and on our bodies as we do human cells. For decades biologists estimated that we had about 10 times as many microbial cells as our own. But a new study found that the average man has about 39 trillion bacteria in his body and about 30 trillion human cells. That’s still more than half of “me” that’s not what I think of as “me”—and it doesn’t count all of the viruses, fungi, archaea, and other life forms that make up what’s called the human microbiome.

We’re covered in them. We’re full of them. They’re in our guts. They’re all over our skin.

Sound gross? Don’t think of them as germs. These trillions of creatures that make up the human biome are important parts of almost every area of our lives. They shape how and what we eat, how we reproduce, how we fight disease, how we sleep. From our earliest moments, they are very much us. ...

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