But traditionally the study of history hasn't focused much on diseases, partly because they're mysterious and terrifying and partly because they don't fit in very well with our ideas about history being the result of human agency.
But, in fact, history often happens because lots of people got smallpox. There's also the fact that diseases were often seen to be the result of divine will or else divine wrath.
Like Ancient Rome's immigration into trans-continental trade networks like the silk road may explain why the historian Livy reported at least eleven pestilential disasters and it is very likely that disease and the accompanying decline in population contributed to the fall of the Roman Empire.
But anyway the black death started in China and spread westward over the caravan routes, really picking up steam when plague-carrying rats jumped onto Mediterranean trading ships. So the death rate from this plague was incredibly high, perhaps a third of people living in Europe died.
So, diseases have been with humans as long as there have been, like, humans. And humans first appeared in tropical regions in Africa which are home to a wide variety of micro-parasites so it's probably a good bet that those parasites played a role in keeping human populations really low for a long, long time.
Also, River Valleys can be breeding grounds for disease, especially were cultures developed irrigation which often relied on slow moving or standing water. If you ever had to clean a bird bath, you'll know that standing water is the perfect environment for disease carriers and nasty microorganisms.
So we are not 100% sure that the "Black Death" was Bubonic Plague. Its virulence suggested in some places it might have been pneumonic, but we have descriptions of it that match Bubonic Plague, like this one from Florentine chronicler Matteo Villani "it was a plague that touched people of every condition, age and sex.