structural:summarization, quality:corruption
Population decline

After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly. This was largely the result of the epidemics of viruses brought to the continent against which the natives had no immunity. In 1520–1521, an outbreak of smallpox swept through the population of Tenochtitlan and was decisive in the fall of the city; further significant epidemics struck in 1545 and 1576.

There has been no consensus about the population size of Mexico at the time of European arrival. Early estimates gave very small population figures for the Valley of Mexico, in 1942 Kubler estimated a figure of 200,000. In 1963 Borah and Cook used preconquest tax lists to calculate the number of residents in central Mexico, estimating over 18–30 million. Their very high figure has been highly criticized for relying on unwarranted assumptions. Archeologist William Sanders based an estimate on archeological evidence of dwellings, arriving at an estimate of 1–1.2 million inhabitants in the Valley of Mexico. Whitmore used a computer simulation model based on colonial censuses to arrive at an estimate of 1.5 million for the Basin in 1519, and an estimate of 16 million for all of Mexico. Depending on the estimations of the population in 1519 the scale of the decline in the 16th century, range from around 50 percent to around 90 percent – with Sanders's and Whitmore's estimates being around 90 percent.