[wmiel]

The Spanish flu largely overlapped with the WW1, and since its 2nd wave had a high toll among people with strong immune system (ex. young male) the perception was to a large degree mixed with the perception of war and its casualties, at least in Europe.

The name 'spanish flu' itself also stems from the fact that most countries involved in the war didn't want to talk too much about the Flu, while Spain retained neutrality and didn't have incentives to keep it in the dark - this could also be a reason that people were putting the aftermath of the pandemic in the same bucket as the outcome of the Great War imo.

https://www.history.com/topics/world-war-i/1918-flu-pandemic