San Francisco had the 1918 flu under control. Then it lifted restrictions.

By the end of October, there were 20,000 cases and more than 1,000 deaths. However, as the days went on, the city saw a dip in newly reported cases, which prompted officials to begin to reopen the city and rescind the mask order. By the end of November, officials believed the city had stabilized.

But three weeks after that celebration of removing their masks, the city saw a dramatic resurgence. Officials at first rejected the idea of reopening the city and suggested residents could voluntarily wear face coverings.

But shortly after the New Year in 1919, the city was hit with 600 new cases in one day, prompting the Board of Supervisors to re-enact the mandatory mask ordinance. Protests against the mandate eventually led to the formation of the Anti-Mask League. The detractors eventually got their way when the order was lifted in February.