An image of recreated 1918 influenza virions. Courtesy of the Center for Disease Control. More info
Almost as quickly as the influenza pandemic appeared in the spring of 1918, the rate of new cases slowed by the end of 1918, then faded away by the spring of 1919. The immediate flu crisis had passed but Americans would deal with its consequences long after. Estimates vary, but the most widely accepted world death tolls reach 25 to 50 million worldwide, with more than 1.5 million of these on American soil, in addition to military losses abroad.
Other pandemics and health crises affected early twentieth-century Americans and their predecessors, but this one was different because of its rapid spread, faster progression, and disproportionate effect on the young and healthy. This, combined with the loss of so many young men at the front to disease and injury, led to a population lull in the US that would take several years to rebound to original numbers. The disease itself was so virulent that it continued to mutate and resist eradication. In fact, all Influenza-A outbreaks since that time have been descendants of the 1918 strain.