The arrival of Europeans of the Old World to the New World brought forth a transatlantic trade referred to as the Columbian Exchange. This was a wide transfer of foods, livestock, disease, and culture in the 15th and 16th centuries that truly changed the course of human history. To the Old World came goods such as tomatoes, maize (corn), and turkey, while to the New World, cows, horses, and sugarcane were introduced (these are only some of the many exchanged goods of the Columbian Exchange).
However, what many scholars deem as the most important and devastating was disease. Native Americans (including Mesoamerican and South American inhabitants) had not been in contact with New World diseases such as smallpox, measles, cholera, malaria, or typhus. Epidemics wiped out entire civilizations and resulted in the genocides of an estimated 40-50 million.
On another note, Spanish conquistadors took over the pre-Columbian world. Hernán Cortés conquered the Aztecs while Francisco Pizarro subdued the Incas. Both were aided with advanced with guns and steel weapons, but mainly with disease. A third of the Aztecs died due to smallpox, and the Incans saw a 93% declination in their population.
