The 1918 flu pandemic caused the excessives of the 1920’s by redistributing and concentrating wealth - from the elderly (who died in large numbers) to the healthy and financially gullible young.
The way I heard it, the flu hit (and killed) young healthy adults worse than any other category. Something to do with immune system overreaction.
But war and disease certainly had its consequences.
So did motion pictures. Sudden wealth in Hollywood led to excesses onscreen and off, and the films affected fashion and morals.
Also, war got women out of the house and into the work force.