This Valentine’s Day, as you bask in the beauty of your beloved, don’t just thank his or her genes and your good fortune; thank microbes. 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.