In one sense, you are more bacteria than you are human. At least, you are vastly outnumbered by the amount of bacteria inside you. The human body contains 10 times more bacterial cells than human cells. Advances in DNA technology are finally allowing scientists to study these microbial companions in depth, deepening our understanding of how the colonies of bacteria that coexist with the human body—the microbiome— influence mental and physical health. Each one of us has an utterly unique melange of different types of bacteria that can affect our anxiety and depression levels, influence our weight, and serve as protection against disease, among other things.