Final answer:
Old World diseases like smallpox brought by Spanish conquerors played a significant role in the downfall of the Aztec and Inca civilizations, as the New World natives had no immunity and suffered catastrophic population losses, which facilitated Spanish conquest and colonization.
Step-by-step explanation:
Impact of Old World Diseases on the Aztec and Inca Civilizations
The Old World diseases, such as smallpox, had a catastrophic impact on the Aztec and Inca civilizations during the Spanish conquest. The natives of the New World had no previous exposure and thus no immunity to these diseases, resulting in extremely rapid and widespread epidemics. For example, smallpox drastically reduced the Aztec population right when they were gaining the upper hand against the Spanish. This weakened the indigenous populations, leading to social chaos and military weakness which the Spanish exploited. In a short span of time, diseases facilitated the Spanish conquest as they took over major cities and empires, such as Tenochtitlán and Cusco, and entrenched themselves in the power structures of these civilizations. The population decline was severe, and within decades, millions of natives had died, causing a demographic collapse that is considered one of the most severe in human history. The Columbian Exchange not only involved the transfer of crops, animals, and resources but also these deadly diseases, which had long-lasting effects on the populations and cultures of the Americas.