A very good explanation of the “Spanish” flu can be found here:
https://www.youtube.com/watch?v=sE0-WWqhFls&t=186s&frags=pl%2Cwn
The group that produced this is also doing a YouTube week-by-week history of WWII. Their latest is for the week of June 1, 1940. Indie Nidell, the narrator also did a week-by-week history of the Great War which ended in Nov. 2018
mark to watch