The Influenza Pandemic of 1918 https://virus.stanford.edu/uda/
The influenza pandemic of 1918-1919 killed more people than the Great War, known today as World War I (WWI), at somewhere between 20 and 40 million people. It has been cited as the most devastating epidemic in recorded world history. More people died of influenza in a single year than in four-years of the Black Death Bubonic Plague from 1347 to 1351. Known as “Spanish Flu” or “La Grippe” the influenza of 1918-1919 was a global disaster.
https://virus.stanford.edu/uda/
One interesting thing about the 1918 pandemic is that it hit healthy young adults the hardest.
Typically, pandemics kill the weakest. 11918 wiped out the very group that would normally be able to reproduce and revitalize the economy.
The technical reasons are a bit beyond me, it had to do with the fact that the healthier the immune system the more it overreacted.
So its effect in economic and population terms was vastly wors than anything before or since