Still, the fact that we had better medicine, along with the fact that people 100 years ago didn't throw their excrement out of their 2nd floor windows, along with hygienic handling of corpses beginning to appear, should have somewhat cut down the death rate.
Spanish Flu was a bit of a mystery illness to the world by the standards of any time. It wasn't affecting the young, old or sick.
Instead it was hitting and killing healthy people in the prime of their life.
Furthermore it spread on a massive scale due to the ending of WW1. There had never been such a huge amount of international transport in human history before then. It spread like wildfire.
Had it, or a similarly infectious disease which affects people in such a way, appeared today, it would still do great harm. Particularly in the third world, where education, sanitation, hygiene and disease control are lacking.
And if the more developed world didn't have stricter measures for disease control then it would easily spread.
