Plague of Justinian
The Plague of Justinian was the first major outbreak of plague in the Mediterranean Basin. It severely affected Europe and the Near East (e.g. the Sassanian Empire), with written records particularly documenting its effect on the Byzantine Empire (or Eastern Roman Empire). Plague arrived in Roman Egypt in 541, spread outwards from there, and persisted in northern Europe and the Arabian peninsula until 549. There continued to be recurrences of plague for two centuries. It’s estimated that about 25% of the population of the eastern Mediterranean and 40% of the city of Constantinople died in the initial pandemic, with between about 15–100 million dying of plague over the course of the two centuries following the initial outbreak.
The plague had a huge impact on the course of European history at the time. It severely weakened the Byzantine Empire at a moment when they had just reconquered nearly all of Italy, meaning that they could not hold on to this conquered land for very long, with northern Italy falling to the Lombards in 568. As such, it cemented the division between the eastern (Byzantine) and western (disintegrating) halves of the Roman Empire. The plague was also a factor in the language shift of the Balkans from Romance and Greek to Slavic languages; the high mortality rate among Byzantine citizens in the peninsula meant that they were rapidly overwhelmed and outnumbered when Slavs migrated to the area from the northeast.
It is thought that the viral strain responsible for the Justinian plague originated in Tian Shan, a mountain range straddling the border of China, Kazakhstan and Kyrgyzstan. The Black Death, centuries later, was not a direct descendent of the Justinian plague strain.