Research on the microbes that inhabit our bodies has progressed rapidly in recent years. Scientists think that these communities, most of which live in the gut, shape our health in myriad ways, affecting our vulnerability to allergic diseases like hay fever, how much weight we put on, our susceptibility to infection and maybe even our moods. They can also, it seems, make us sexy . Susan Erdman, a microbiologist at MIT, calls it the “glow of health”. The microbes you harbour can make your skin smooth and your hair shiny; they may even put a spring in your step. She stumbled on the possibility some years ago when, after feeding mice a probiotic microbe originally isolated from human breast milk, that the animals grew unusually lustrous fur. What Dr Erdman’s research suggests is that the microbes that make us attractive to potential mates, also directly influence our reproductive success. So when mammals choose mates based on the glow of health, they’re choosing not just an attractive set of genes, but also a microbial community that might facilitate reproduction. Another way to look at it: By making their hosts sexy, and by increasing hormones that bring mammals together, microbes help to ensure their own continued existence by the creation of another host. “Everyone wins,” Dr Erdman says.