The so-called Spanish Flu (which actually originated in the United States) first hit in the spring of 1918. It went away over the summer, only to come back in the fall, more deadly than before. One of the cities hardest hit was Philadelphia, which saw up to 759 people die in a single day in October. And one of the big factors in the city’s death toll was a massive World War I parade held on September 28 that drew around 200,000 people.

“Within 72 hours of the parade, every bed in Philadelphia’s 31 hospitals was filled,” Kenneth Davis wrote in Smithsonian in 2018. “In the week ending October 5, some 2,600 people in Philadelphia had died from the flu or its complications. A week later, that number rose to more than 4,500.”