What was the main cause of the death of 20 million natives in central Mexico? (1450-1550)

The population figure of indigenous peoples of the Americas before the 1492 Spanish voyage of Christopher Columbus has proven difficult to establish. Scholars rely on archaeological data and written records from European settlers. Most scholars writing at the end of the 19th century estimated that the pre-Columbian population was as low as 10 million; by the end of the 20th century most scholars gravitated to a middle estimate of around 50 million, with some historians arguing for an estimate of 100 million or more.[1] Contact with the Europeans led to the European colonization of the Americas, in which millions of immigrants from Europe eventually settled in the Americas.

The population of African and Eurasian peoples in the Americas grew steadily, while the indigenous population plummeted. Eurasian diseases such as influenza, pneumonic plagues, and smallpox devastated the Native Americans, who did not have immunity to them. Conflict and outright warfare with Western European newcomers and other American tribes further reduced populations and disrupted traditional societies. The extent and causes of the decline have long been a subject of academic debate, along with its characterization as a genocide.[2]