WestYorksRoyal The Spanish Flu
In today's peacetime scenario, there's no knowing what impact it would have had, but realistically it would have been a fraction of what actually happened.
There than the debated war-element
We are without a vaccine, India, Pakistan, Bangladesh, Africa without significant health resources, and the world population is 3 times the size. So, if we eventually this pandemic was only one third as deadly as 1918 (in terms of percentage deaths, that could still be 20-50 Million