Civilization picked up speed after the discovery of agriculture and animal husbandry, allowing for longer lifespans, healthier people, and lots of population growth. Unfortunately, with population growth comes denser living quarters, enabling contagious diseases to spread more quickly. As humankind spread across the globe in waves of migrations, so did epidemics of unfathomable destruction.
The era of plagues arrived as communities of dense human populations grew. Plague after plague destroyed entire cities at levels inconceivable today.These epidemics figured in to the fall of entire empires: The Han Dynasty in China, the Roman Empire, and the Aztecs were all pushed towards extinction in great part by epidemics that wiped out up to 90% of their populations. (90%!!!)