Answer:
The spread of epidemic diseases in Eurasia during the fourteenth century mostly refers to the bubonic plague that hit East Asia, the Middle East, and Europe during this period.
The plague, carried in the fleas of rats, took several years to reach Europe from East Asia, mostly following the Silk Road. In Europe, in entered through Italy, and 5 years after, it had reached Northern Europe. It killed about a third of total European population at the time, causing deep changes in European society and economy.
The spread of epidemic diseases int he Americas during the sixteenth century was even more lethal, because it killed around 90% of Pre-Columbian Population. It was caused by the spread of diseases like measles and syphilis, which were common in Eurasia, but unknown in the Americas, meaning that Native American populations had no antibodies to fight these diseases.
These epidemics were the definitive factor that tipped the balance of power in the Americas, in favor of Europeans. Only a few fringe regions of the continent, mainly in the extreme north and south, were spared.
