In 1918 Philadelphia US threw a parade to support the war effort that drew 200,000 people who crammed together to watch the procession. Three days later, every bed in Philadelphia’s 31 hospitals was filled with sick and dying patients, infected by the Spanish flu.

A different story played out in St. Louis, just 900 miles away. Within two days of detecting its first cases among civilians, the city closed schools, playgrounds, libraries, courtrooms, and even churches. Work shifts were staggered and streetcar ridership was strictly limited. Public gatherings of more than 20 people were banned.

The extreme measures—now known as social distancing, kept per capita flu-related deaths in St. Louis to less than half of those in Philadelphia, according to a 2007 paper in the Proceedings of the National Academy of Sciences.