From asthma to Crohn’s disease, everyone knows someone who suffers from an allergic or autoimmune disorder. And if it appears that the prevalence of these maladies has increased recently, that’s because it has—to levels never before seen in human history. These days no fewer than one in five—and likely more—Americans suffers from one of these ailments. We seem newly, and bafflingly, vulnerable to immune system malfunction. Why? One possibility is that we have systematically cleaned ourselves to illness; this belief challenges deeply entrenched notions about the value of societal hygiene and the harmful nature of microbes.