Biggest death rates in ancient times were due to child mortality and maybe communicative diseases and hunger aftet that. Gemora Taanis suggests start davening when a pandemic of people or animals, even pigs, happens in areas connected to your city by trade routes.
And when pandemic reaches your city, then run away or go into lock down.
And due to child and hunger mortality, probably wealthy adults were stronger and healthier than some now.
As to overall effect, just notice billions of people on the planet with decreased birth rates and figure it out.