Well, it;'s a matter of common sense. The vast majority of diseases that killed so many came from unsanitary conditions and a lack of understanding of what it took to cure them. The Black Plague is one. It came at a time when people threw their garbage in the streets and lived close together in horrid conditions. Rats picked up infected fleas that passed it on to humans. We also have far healthier diets today. But, that also means we have less immunity against diseased if we don't vaccinate so that when they do appear, we are more likely to become infected.
Diptheria breaks out in Rohingya refugee camps... Rare Disease Finds Fertile Ground In Rohingya Refugee Camps January 27, 2018 - Diphtheria poses one more threat to already beleaguered Rohingya refugees.