“The gut microbiome has transformed our understanding of human health,” says Tim Spector at King’s College London, co-founder of the Zoe nutrition app. “We now know our microbes influence everything from metabolism and immunity to mental well-being.” While this understanding has accelerated over the past 25 years, humans have long used microbes to influence health. While they didn’t realise what they were doing, the Romans used bacterial-derived remedies to “guard the stomach”, for instance. In the 17th century, microbiologist Antonie van Leeuwenhoek first described the parasite Giardia, from his own stool. Yet it took another two centuries for scientists to validate his findings, and until the 21st century to appreciate how deeply the microbes that line our guts and skin affect our well-being. By the 1970s, researchers were making headway, realising that gut microbes could influence how drugs are broken down, altering their efficacy. Experiments with faecal transplants hinted at how microbial communities might restore health. But it was only in the 2000s, with rapid advances in genetic sequencing and computing, that the field was truly transformed. Early genome sequencing revealed …