How the refusal to ban large public gatherings during the 1918 Spanish flu pandemic led to disaster. Here's everything you need to know:

Why hold a parade?
America was in the waning months of World War I, and officials across the country were under enormous pressure to sell war bonds, or Liberty Loans. Big parades were staged in major cities to rally the public behind the war bond effort. On Sept. 28, 1918, Philadelphia city officials refused to cancel their parade amid the Spanish flu pandemic, with devastating health consequences. That decision has been held up by the Centers for Disease Control as an example of what not to do during a pandemic, and has evoked obvious parallels with some modern-day officials' refusal to implement disruptive shelter-in-place orders and cancel events. New York City Mayor Bill de Blasio and New York Gov. Andrew Cuomo, for example, initially refused to heed calls from health experts to shutter the city's sprawling public school system and take other steps to minimize public gatherings. "It seemed to me to be a perfect parallel to the story of what happened in Philadelphia in 1918, where the health authorities were clearly aware that this was a growing problem," said historian Kenneth C. Davis.

Why didn't Philadelphia cancel the parade?
The decision fell to Health Commissioner Wilmer Krusen, a political appointee with no prior public health experience. Doctors pleaded with him to cancel the parade, with one branding it "a ready-made inflammable mass for conflagration." The Spanish flu was a new strain of the influenza virus to which no one had any immunity, and like today's coronavirus, it often led to pneumonia and an overreaction by an overwhelmed immune system — a "cytokine storm" — that destroyed people's lungs. It gained a foothold among soldiers in the trenches of Europe, and would eventually infect a third of the world's population and kill an estimated 675,000 Americans and 50 million people globally. At the time Krusen was making his decision, Boston was already suffering the consequences of holding its own "Win-the-War-for-Freedom" Parade on Sept. 3. By Sept. 23, The Boston Globe reported that the city's hospitals were "taxed to their limits." At the time the Philadelphia parade was held, 600 sailors and 47 civilians had been diagnosed with the flu at the Philadelphia Naval Yard, and some had already perished. In St. Louis, City Health Commissioner Dr. Max C. Starkloff faced the same decision as Krusen — and canceled his city's Liberty Loans parade.

How did Krusen react?
In the days before the parade, Philadelphia officials distributed 20,000 flyers urging citizens to cover their mouths when they coughed or sneezed. "If the people are careless," Krusen was quoted as saying in the Phil­a­del­phia Even­ing Bul­le­tin the day of the parade, "thousands of cases may develop and the epidemic may get beyond control." That same article reported that 118 people in the city had been diagnosed with the virus. But still he permitted the parade to proceed. On the 28th, a snaking line of Boy Scouts, marching bands, women's auxiliaries, and troops 2 miles long wound its way up Broad Street. War­planes flew overhead as an enormous, tightly packed crowd of 200,000 cheered. Within three days, every bed in Phil­a­del­phia's 31 hospitals was occupied. Within a week, 45,000 citizens were infected and the entire city had shut down.

Did the shutdown help?
It was too late. By the second week in November, 12,000 Phila­del­phians were dead, and the phrase "bodies stacked like cordwood" had become commonplace among the survivors. "It was an apocalyptic scene," said Davis. "In some cases, public-health nurses would be walking into tenements and finding whole families dead." Bodies piled up on sidewalks after the city morgue, capable of holding only 36 people, was overwhelmed. Within six months, 16,000 were dead, and 500,000 Phil­a­del­phians had fallen ill with the flu. Mean­while, unsubstantiated rumors circulated among the frightened populace that the Germans had unleashed the flu on U.S. soil via spies who'd arrived on U-boats.

How did other cities fare?
Researchers have found that cities that quarantined the sick and shut schools, churches, and theaters saw 50 percent lower death rates than those that did not. In Milwaukee, which had the lowest death rate (0.6 percent) of any large city in America during the pandemic, the city's health commissioner, Dr. George Ruhland, had aggressively shut schools, saloons, and public places the moment the virus arrived there, and plastered the city with an ad campaign warning people to stay home. Even after the restrictions were lifted, dance-hall revelers on New Year's Eve still wore six-layer gauze masks as a precaution, with the Milwaukee Sentinel describing them as looking like "a band of holdup men from the neck up." In St. Louis, where Starkloff canceled the parade, the peak death rate was only one-eighth that of Philadelphia.

The economic impact of shutdowns
A study published this year argued that cities that acted early and aggressively to impose social distancing to limit the spread of the Spanish flu actually performed better economically after the pandemic was over than those that did not. Fewer workers had died, and the local population more quickly resumed normal economic behavior, three economists found. "It casts doubt on the idea there is a trade-off between addressing the impact of the virus, on the one hand, and economic activity, on the other hand, because the pandemic itself is so destructive for the economy," said lead researcher Emil Verner of the Massachusetts Institute of Technology. A very similar controversy is playing out today, as some Republican governors refuse to issue stay-at-home orders and shutter businesses, arguing that the economic damage of social distancing would be worse than the disease itself. "The people themselves are primarily responsible for their safety," said South Dakota Gov. Kristi Noem, one of seven governors who have refused to issue statewide stay-at-home orders. (Her state has more than 800 cases of Covid-19.) Some have justified their inaction by citing President Trump's insistence that this should be a state decision. "I leave it up to the governors," Trump said, adding they "know what they're doing."

This article was first published in the latest issue of The Week magazine. If you want to read more like it, try the magazine for a month here.