The most interesting thing I learned in researching both pandemics was that each was preceded by a massive volcano--in both instances from present-day Indonesia.
It also appears that both pandemics (The Plague of Justinian in the sixth century and The Black Death in the fourteenth century) were caused by the same pathogen: Yersinia pestis (Y. pestis).
So the two most deadly plagues in recorded history were both caused by the same pathogen and BOTH were directly preceded by a massive volcanic eruption in present-day Indonesia.
Both plagues were incredibly important--historically. The Plague of Justinian in many ways led to the fall of the ancient world and the dawn of the dark ages. The population decline and the weakening of the world's greatest empires (including The Eastern Roman Empire and Persia) ended a more or less constant march of progress across tens of thousands of years since the Toba Catastrophe.
Here are some details on both pandemics:
The Plague of Justinian in the sixth century was the first "super pandemic" in recorded
human history. There's very strong evidence that this pandemic was the direct result of the Indonesian volcano Krakatoa. But the jury is still out... See this link for more
The Black Death in the fourteenth century, gets a lot more press, mostly because there are more details about it in the historical records. We know that it was incredibly similar to The Plague of Justinian (same pathogen, same point of origin, supposedly: China).
A recent discovery really tied the entire volcano-plague thesis together and gave me a great opening with the plot. Researchers now believe a volcano in Indonesia around 1257 led to widespread famine and a volcanic winter across Europe:
The Black Death occurred almost a hundred years later, but... that was close enough for me!