I have a theory, but first it helps if you take off your clothes. Go ahead and do it in the privacy of your bedroom. Actually, I don’t care where you do it. Your kitchen is fine, even your living room. Stand there, and consider your life. You are alone.

Well, almost. There are the mites that live on your forehead. You’re also shar­ing your space with the hundreds of bac­teria species that live in your gut, mouth and everywhere else. Actually, the bacte­rial cells living on—and in—your body probably outnumber your own cells. Oh, and don’t forget about the fungi that live in your lungs and hair. Now that I think about it, you may even have a worm or two sloshing about in your intestines. Many of us do. The truth is that even when naked, showered and scrubbed clean, you have company. Thousands of species can be found on an average, living, human body—yours, mine, even the person who bumps into you accidentally on the bus. You are never alone.

The surprising thing about the wild variety of life that lives on us is that, until very recently, we didn’t realize it was there. Until even more recently we thought it was, well, bad—germy, the sort of stuff we should wipe off. More to the point, even when we know a species lives on us, we don’t typically know very much about it. Nothing, really. I would argue that our bodies are one of the least understood hab­itats on Earth, one of the last great fron­tiers. Why, when each year billions of dol­lars are spent to research humans, are we still so ignorant about the species that live on us? I have a theory.

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