The black plague managed to wipe out a third of Europe long before the advent of modern transportation technology.
"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.
It's estimated it wiped out 25-60% of the population of Europe over the course of 2 centuries (15-100 million people).