The 1918 Spanish Flu had about a 10% fatality rate worldwide and about 2.3% in the US. See the Wikipedia article. About 30-35% of the population in most countries caught it - 500 million of 1400 million people worldwide = 35.7%, and 50 million died.
“What Explains the Difference Between Estimated COVID-19 Fatality Rates in New York and California?”
I know, finding accurate data from that time is exceedingly difficult. They did not keep great records at the time. The CDC did not exist yet to keep records on every single death in the US.