Complex sentence: Unsanitary conditions and overcrowding were widespread throughout Europe and Asia during the Middle Ages, resulting in cataclysmic pandemics such as the Plague of Justinian (541-42) and the Black Death (1347--1351), which killed tens of millions of people.
Split into simpler sentences: Unsanitary conditions and overcrowding were widespread throughout Europe and Asia during the Middle Ages. This resulted in pandemics such as the Plague of Justinian (541-42) and the Black Death (1347--1351), which killed tens of millions of people.