I’m curious: was the Spanish flu the first flu pandemic, or were there other versions before that. If there were previous flus, some folks might have had a partial immunity, making the virus’ ability to spread more difficult.
Influenza had been around for a long time. By 1918 it was very well known and the physicians of the day immediately identified the Spanish Flu as a new variety of influenza. Read the book. It is very informative.