In 1918, there was uncertain hygiene, a poor understanding of routes for contagion, no antivirals, no time or method to make a vaccine, and a lot of people had chronic diseases (tuberculosis, diabetes poorly treated, syphilis, etc.). Also many more people smoked cigarettes.
That's mostly true, but not true about cigarettes. See:
https://www.nap.edu/read/11795/chapter/4#42