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.”
The parade went forward despite warning signs because the city’s director of public health didn’t want to cause a panic and because he was under pressure from federal officials to meet bond quotas. “Caught between the demands of federal officials and the public welfare, he picked wrong,” Davis wrote. Another factor was President Woodrow Wilson's effort to cover up the seriousness of the flu—he wanted people focused on the war effort.
Philadelphia’s leaders shut down the city days after the parade, but it was too late, especially with many doctors having been drafted into the war. The medical system was overwhelmed and morgues couldn’t keep up. As is starting to happen in some U.S. cities now.