“Hot” new areas of biomedicine sometimes generate cool skepticism. Little more than a decade ago, investigators proposed that the gut microbiome might be contributing to obesity. Since then, the microbiome has been linked to numerous major diseases, including atherosclerosis, although some have been skeptical about this association.

How could the gut microbiome influence the course of any disease? The central argument is simple and compelling. Humans actually have 2 genomes: human genes and the collective genes (the “microbiome”) of the trillions of microbes (the “microbiota”) that coexist with each human.