On Sept. 28, 1918, in the waning days of World War I, over 200,000 people gathered along Broad Street in Philadelphia for a parade meant to raise funds for the war effort. Among the patriotic throngs cheering for troops and floats was an invisible threat, which would be more dangerous to soldiers and civilians than any foreign enemy: the influenza virus.
Officials went ahead with the parade despite the discouragement of the city health department about the ever-spreading virus. Within 72 hours of the parade, all the hospital beds in Philadelphia were full of flu patients. Within six weeks, more than 12,000 people died — a death every five minutes — and 20,000 had died within six months.