Final answer:
The Crusades led to increased trade which allowed rats carrying fleas infected with the bubonic plague, otherwise known as the Black Death, to spread the disease across Europe, resulting in the deaths of nearly half the population.
Step-by-step explanation:
The Crusades indeed led to a resurgence of trade, which facilitated the movement of rats carrying fleas infected with the bubonic plague. The Black Death, as it came to be known, devastated populations across Europe, killing nearly half of the population. Increased trade routes including the Silk Roads and the Mediterranean Sea allowed for the rapid spread of the plague, as rats became stowaways on ships and in merchant caravans. The link between trade, the movement of rats, and the spread of disease is clear, with historical evidence suggesting that the outbreak was worsened by the movement of Mongol armies and refugees amidst environmental and famine conditions, further facilitating the spread of the disease. The Black Death led to significant changes in European society, including challenges to the feudal system and setting the stage for the rise of capitalism.