Exactly 100 years ago this summer, New York City was hit by one of the worst epidemics it had ever seen: the first explosive American outbreak of infantile paralysis — a disease later known as polio.

It was a mystifying, terrifying illness. Most victims were toddlers. Initially, they didn’t even look terribly ill; often, they woke up feverish and cranky, saying their necks hurt.

Soon, however, instead of climbing out of bed, they were able only to drag themselves along the floor, legs flopping behind. Within a few hours, in some cases, they would struggle to breathe, and then die.