[soiler]

But long after the taming of horses, the invention of roadways and carts, and the proliferation of dense urban areas.

"International" trade has surely always existed - we know people far from the sea decorated their bodies with cowrie shells in prehistoric ages. But the scale of contact and interaction has not remained constant.

Arguably, the Middle Ages is when Eurasian society was at its weakest WRT disease.

[yareally]

The Plague of Justinian (the first major instance of the disease) in the 6th Century wiped out at least 40% of the population of Constantinople--the largest and most advanced city in Europe at the time.

It's estimated it wiped out 25-60% of the population of Europe over the course of 2 centuries (15-100 million people).

https://en.wikipedia.org/wiki/Plague_of_Justinian