One hundred years ago, antibiotics revolutionized medicine. Bacterial infections were wreaking havoc, and the emergence of a drug capable of annihilating microbes led to an unprecedented improvement in life expectancy. Since then, the massive — and sometimes negligent — use of antibiotics has allowed bacteria to adapt to its attacks and antibiotic resistance is threatening to devalue medicines that have saved millions of lives. Now, the study of the intestinal microbiota, an ecosystem of microor...