American Plagues: 16th century

The American Plagues are a cluster of Eurasian diseases brought to the Americas by European explorers. These illnesses, including smallpox, contributed to the collapse of the Inca and Aztec civilizations. Some estimates suggest that 90% of the indigenous population in the Western Hemisphere was killed off.  The diseases helped a Spanish force led by Hernán Cortés conquer the Aztec capital of Tenochtitlán in 1519 and another Spanish force led by Francisco Pizarro conquer the Incas in 1532. The Spanish took over the territories of both empires. In both cases, the Aztec and Incan armies had been ravaged by disease and were unable to withstand the Spanish forces. When citizens of Britain, France, Portugal and the Netherlands began exploring, conquering and settling the Western Hemisphere, they were also helped by the fact that disease had vastly reduced the size of any indigenous groups that opposed them.

(Source: https://www.livescience.com/worst-epidemics-and-pandemics-in-history.html )