Why the microbiome–our rich inner ecosystem of microorganisms–may hold the keys to human health.We are at the dawn of a new scientific revolution. Our understanding of how to treat and prevent diseases has been transformed by knowledge of the microbiome–the rich ecosystem of microorganisms that is in and on every human. These microbial hitchhikers may hold the keys to human health. In Gut Feelings, Alessio Fasano and Susie Flaherty show why we must go beyond the older, myopic view of microorganisms as our enemies to a broader understanding of the microbiome as a parallel civilization that we need to understand, respect, and engage with for the benefit of our own health.