In September of 1918, soldiers at an army base near Boston suddenly began to die. The cause of death was identified as influenza but it was unlike any strain ever seen. As the virus spread across the country, hospitals overfilled, death carts roamed the streets, and helpless city officials dug mass graves. It was the worst epidemic in American history, killing over 600,000—until it disappeared as mysteriously as it had begun.

[text adapted from pbs.org]