Final answer:
Prior to the Columbian Exchange, infectious disease exposure was more common in Europe primarily due to Europe's larger population. This led to a level of immunity in Europeans, while Native Americans, having no prior exposure, were decimated by the newly introduced diseases with up to 90 percent dying after European contact.
Step-by-step explanation:
Prior to the Columbian Exchange, exposure to infectious disease was more common in Europe than the Americas primarily due to Europe's larger population. The larger populations in European urban centers meant that diseases could be transmitted more easily and that Europeans were continually exposed to a variety of diseases, leading them to develop a certain level of immunity over generations. In contrast, Native Americans had no natural resistance to European diseases, such as smallpox and measles, due to a lack of prior exposure, particularly to diseases originating in European domesticated animals.
When European explorers reached the Americas, they unwittingly initiated epidemics that devastated Native American populations. This was not because of superior hygienic practices in Native American societies or the use of inoculation, which they did not have, but because Native Americans had never encountered these diseases before. As a result, up to 90 percent of Native Americans died from infectious diseases after the arrival of Europeans, fundamentally altering the demographic landscape of the New World and enabling European conquest.