Hidden deep in our intestines, 100,000 billion bacteria are keeping us healthy by producing a range of molecules. Although their names may be perplexing — Fecali bacterium, Roseburia, Akkermansia mucinifila, Eubacterium halli... — and they are invisible to the naked eye, they could revolutionize the future of medicine. That is, if our modern lifestyle doesn't wipe them out first.