In some cases, the same diseases were responsible for multiple instances of plague, and it's hard to accurately diagnose each of them hundreds or even thousands of years later. "Plague" killed Pericles and a host of others when Athens was besieged in the Peloponessian War; WHICH plague remains an open question.

I don't agree the Romes plague near the end is what killed it though; overextension and complacency during the Pax are more attractive candidates. Christian unity and the assimilation of so many "barbarian" German tribes kept Rome on life support and hid the decline for at least a century after its peak, but had the Empire been sustainable Constantinople would never have been founded.