Martin Blaser, professor of medicine and microbiology at New York University, has a theory about why humanity is getting fatter. He believes that some of our modern medical practices are to blame. The main culprit? Antibiotics – or rather, the way we have abused these antibacterial drugs over the last 70 years. Each one of us hosts a collection of good bacteria, known as the microbiome, that keep us healthy by training our immune system, helping us digest food and making essential vitamins for us. In this episode, Martin Blaser explains how this useful alliance is perturbed when we take antibiotics. Who would have guessed that the same ‘miracle drugs’ that cure us from horrible, deadly infections would – in some ways – make us sicker?