Some cleaning agents claim to kill 99.9% of all bacteria, but if preventing disease is the main aim, then maybe total annihilation isn’t the best course to take. We discuss this and other lessons we have learnt by studying the microbiome: the microbes that live on and within our bodies. Hear about the man who had a literal window into his digestive system, and how releasing the right kind of mosquito can prevent the spread of dengue fever.