For most of history, poliomyelitis was a relatively unremarkable disease – it caused paralysis and occasionally death, but only in a tiny fraction of those infected. It was essentially unknown in infants and adults, and usually only caused mild symptoms in children. This all changed in the early 1900s, when the disease mysteriously transformed into an epidemic, killing many and maiming many more, even among the supposedly ‘protected’ populations of adults and babies.
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 when the weather got warm. Clearly something had changed, but what could cause a mild disease to turn into a killer all but overnight? The secret lies, paradoxically, in our better understanding of sanitation.