A lot of it was war, and the refugees they generate. Trade had a lot to do with the quick spread of disease too.
When the day comes that some really deadly disease with a long enough incubation period to allow sources to be masked at least for a while, the thing that will kill the most people, will be airplanes, because they make the world so much smaller than it used to be.
“A lot of it was war, and the refugees they generate.”
Yes, but my problem is this...and I am not arguing with you...I just need to get to an intellectual bottom of this which has fascinated me for long time: About war: Folks were either on foot or the more lucky few were on horseback. Refugees took longer to move about. The plague hit quick and fast at almost at the same time everywhere.