Great plagues have changed the course of history.
Best known is the Black Death (1347-51), introduced to Europe from Asia through trade routes, carried by rats and transmitted by fleas.
This bubonic plague killed up to one-third to one-half of the population of Europe, almost literally burying the old social order that was medieval feudalism, wiping the slate clean for the rise of mercantilism, the Age of Exploration and the Renaissance.
Lesser known is the Antonine Plague (165-180 A.D.), introduced to the Roman Empire through trade with Asia, probably the measles or smallpox.
The Antonine Plague, according to contemporary accounts, caused 2,000 deaths daily in Rome, an estimated 5 million deaths throughout the Empire, and devastated the Roman legions, hampering defense of the eastern and northern frontiers.
According to Roman Emperor Marcus Aurelius Antoninus, for whom the plague is named, writing from his camp on the Rhine: “With their ranks thinned by the epidemic, Roman armies were now unable to push the tribes back.” (Macromannic Wars)
The Plague of Justinian (541-542 A.D.), introduced to the Mediterranean and Europe from Asia, is one of the deadliest plagues in history, possibly the bubonic plague and rehearsal for the later Black Death. An estimated 25 million to 100 million died, perhaps as much as half the population of Europe.