The Spanish Flu Pandemic of 1918-1920 was the mother of all pandemics.
“The Spanish flu pandemic of 1918, the deadliest in history, infected an estimated 500 million people worldwide—about one-third of the planet’s population—and killed an estimated 20 million to 50 million victims, including some 675,000 Americans.”
https://www.history.com/topics/world-war-i/1918-flu-pandemic
Historians believe that the Black Death was worse than the Spanish flu. Fewer people died overall because there were fewer people on the planet. It took over 200 years for the population to recover.