Final answer:
The bubonic plague, or Black Death, was the main cause of widespread illness and death in Europe between 1250-1500, killing about one-third of the population and being exacerbated by prior periods of famine, economic hardship, and weakened immunity.
Step-by-step explanation:
The beloved causes of illness between 1250-1500, particularly those leading to widespread mortality, include the bubonic plague, also known as the Black Death. This devastating pandemic arrived in Europe after a period of economic contraction, which was marked by famines, crop failures, and malnourishment that preceded the arrival of the plague in the 1340s. The plague was brought to Europe by Genoese merchants returning from the Black Sea and was carried by fleas on black rats. It significantly reduced the population, killing about one-third of Europe's population, with a high mortality rate compounded by poor nutrition and weakened immune systems that made individuals more susceptible to infectious diseases.