Ironically, American soldiers and Sailors who were being sent to save democracy and lives probably spread it. Over the course of a year, three waves of the Flu hammered humanity. I know it's almost unbelievable, and partly because most Americans today know little or nothing about the calamity. Three quarters of a million Americans died, but it doesn't stand out in the history books. Part of the reason is human nature.
War is one thing.
It involves warriors fighting against each other. No matter what weapons they use, the phenomenon is within our area of understanding. Not so with plagues. Too small to see, too deadly to comprehend, too much for the heart and mind to accept, their memory is psychologically repressed by the survivors. It has happened before in history.