This virulent pandemic killed more people worldwide than all of WWI.

This influenza pandemic lasted two years. Symptoms included severe cold, fever, chills, aching bones and muscles.  This virus also generated complications including severe pneumonia, bronchitis, and heart problems.

The Coronavirus and the Great Influenza Pandemic: Lessons from the “Spanish Flu” for the Coronavirus’s Potential Effects on Mortality and Economic Activity
https://www.nber.org/papers/w26866

“Mortality and economic contraction during the 1918-1920 Great Influenza Pandemic provide plausible upper bounds for outcomes under the coronavirus (COVID-19). Data for 48 countries imply flu-related deaths in 1918-1920 of 40 million, 2.1 percent of world population, implying 150 million deaths when applied to current population. Regressions with annual information on flu deaths 1918-1920 and war deaths during WWI imply flu-generated economic declines for GDP and consumption in the typical country of 6 and 8 percent, respectively. There is also some evidence that higher flu death rates decreased realized real returns on stocks and, especially, on short-term government bills.”