I am not getting it either.

"It infected 500 million people around the world,[2] or about 27% of the then world population of between 1.8 and 1.9 billion, including people on isolated Pacific islands and in the Arctic. The death toll is estimated to have been 40 million to 50 million, and possibly as high as 100 million, making it one of the deadliest epidemics in human history"

500 million infected, 40-50 million deaths. Isn't that 10% death rate?

Perhaps it was 2% for people who were treated in hospitals.