In the flu pandemic of 1918, the group it was most dangerous for was the inversion of covid (and normal flu): healthy young people. Our best guess for why this is is that was because it caused a cytokine storm, which is basically an autoimmune response where the body ends up killing itself because the immune system goes way overboard.

Unfortunately while the general idea of cytokine storms had been floating around for quite a while, it wasn't named until 1993, and wasn't really studied as its own thing until 2002/2003. No chance they would have been able to do anything about it in 1918.