Following on from Cheryl's mention of the Spanish Flu, you might be interested in this article in National Geographic:
Social distancing isn’t a new idea—it saved thousands of American lives during the last great pandemic. Here's how it worked.
In 2007, a study in the Journal of the American Medi[c]al Association analyzed health data from the U.S. census that experienced the 1918 pandemic, and charted the death rates of 43 U.S. cities. That same year, two studies published in the Proceedings of the National Academy of Sciences sought to understand how responses influenced the disease’s spread in different cities. By comparing fatality rates, timing, and public health interventions, they found death rates were around 50 percent lower in cities that implemented preventative measures early on, versus those that did so late or not at all. The most effective efforts had simultaneously closed schools, churches, and theaters, and banned public gatherings. This would allow time for vaccine development (though a flu vaccine was not used until the 1940s) and lessened the strain on health care systems.
The studies reached another important conclusion: That relaxing intervention measures too early could cause an otherwise stabilized city to relapse. St. Louis, for example, was so emboldened by its low death rate that the city lifted restrictions on public gatherings less than two months after the outbreak began. A rash of new cases soon followed. Of the cities that kept interventions in place, none experienced a second wave of high death rates.