The human body is, to some extent, just a luxury cruise liner for microbes. They board the SS Homo sapiens when we’re born and settle into their assigned quarters—the skin, the tongue, the nostrils, the throat, the stomach, the genitals, the gut—and then we carry them wherever we go. Some of microbes deboard when we shed our skin or use the restroom; others board at new ports when we shake someone’s hand or down a spoonful of yogurt. Just as on a luxury cruise liner, our passengers eat well. They feed on the food we eat, or on the compounds we produce. While the biggest luxury lines may be able to carry a few thousand people, we can handle many more passengers. Although the total mass of our microbes is just a few pounds, the tiny size of their cells means that we each carry about 100 trillion microbes—outnumbering our own cells by more than ten to one.

Carl Zimmer, “I For One Welcome Our Microbial Overlords”