This is a well written introduction to one of the Roman pandemics. The number of people indicating that they had never heard of this before is interesting.
The end of empires is not simply military defeat and economic failure but, we now begin to understand the integration of those human factors with pandemics and climate change.
The Antonine Plague (smallpox) was devastating but the Roman Empire recovered. There were more coming, however, triggered by the success of trade and by the problems of climate change. Another critical epidemic was the Plague of Cyprian in the 240s CE. The assumption now is that this was another zoonotic filovirus producing a viral hemorrhagic fever. We know this type today as Ebola. It wasted Rome over several years.
The end of the Western Roman empire included a full attack of Bubonic Plague in the time of Justinian, a late peak followed by rapid decline of the Roman Empire in the west.
An excellent study of the combination of disease, climate, and Empire is
The Decline of Rome: Climate, Disease, and the End of an Empire by Kyle Harper.