There used to be a saying, “After war comes plague.” Perhaps the saying still stands, but I suppose the day of jet aeroplane travel has largely made the phenomenon obsolete.
In 1918, as the Great War drew to a close, a deadly influenza spread around the globe. Millions fell victim, strangely not just the elderly and the very young as with most pestilences. Many in the prime of their life also died.
This article on the Smithsonian site tells an interesting tale and dispells many of the myths we have believed for the last hundred years.