Almost three decades ago a British epidemiologist named David P. Strachan proposed a simple, if counterintuitive, idea to explain why hay fever, eczema and asthma had become increasingly common over the preceding century. Strachan linked rising rates of these allergic illnesses in the U.K. with improvements in living standards since the industrial revolution—in particular, a sharp drop in the number of infections experienced in early childhood. He surmised that exposure to bacteria and viruses in the first years of life (provided an infant survived them) somehow protected against these conditions showing up later.