Just how many microbes do each of us host? By one estimate, the human body contains ten times as many microbial cells as human cells, which puts the number easily in the trillions and perhaps in the quadrillions. This “microbial cloud,” as the biologist Jonathan Eisen calls it, is so vast that some scientists consider it the largest organ in the human body. And within it may lie the root of much human health . . . or illness.
