Here’s a puzzler: What if most of the cells in your body are not you? No, that’s not a zen koan, it’s a remarkable way of looking at the nature of what a human body is actually made up of. You see, our bodies play host to perhaps a hundred trillion microbes in the form of bacteria, viruses, fungi, and simple animals. Amazingly, these microorganisms that live in and on our bodies actually outnumber our body’s approximately 10 trillion cells by ten to one. This sounds incredible and perhaps a bit disgusting on first blush, but there’s no getting rid of all of these microbes—and you wouldn’t want to if you could, because many of these organisms are absolutely vital for our health and well being.