For most of history, polio was a relatively unremarkable disease – it caused paralysis and occasionally death, but only in a tiny fraction of those infected. For the most part, it was all but unknown in infants and adults, and only caused mild symptoms in children. This all changed in the early 1900s, when the disease mysteriously transformed into an epidemic, striking down babies and adults with utter unpredictability. Deadly recurrences of polio became a fact of life in developed countries, particularly in cities during the summer. Movie theaters, beaches and swimming pools were closed; families fled to the countryside. What caused this drastic change in an otherwise ‘peaceful’ disease?