The 1918-20 influenza pandemic killed between 15 and 100 million people worldwide, 0.8% to 5.6% in a population of 1.8 billion (see here). Now, with the population 7.8 billion, one of comparable lethality could kill between 60 to 430 million people.
The “Spanish” flu started in Kansas. It spread in 3 main waves. The first one, from March to June 1918, was relatively mild. Soldiers called it “the 3-day flu.” It was seldom fatal, with a mortality rate near 0.5% (5 deaths in a thousand cases), close to seasonal flu of 0.1%.