Cortés’s victory at Tenochtitlán set in motion the rapid collapse of the Aztec empire. Over the next three years, the conquistadores brought the whole of Mesoamerica under Spanish rule and established the colony of New Spain. Smallpox continued to ravage the indigenous population and cripple their capacity to resist the Spanish. Further disadvantaging them was a serious gap in technological advancement. While the Spanish had access to gunpowder and steel, the indigenous tribes armoured themselves with thick cloth and animal hides and fought with macuahuitl (bladed clubs), bows, and spears. These factors working against them, the tribes soon found themselves the subjects of a people arguably as merciless as the Aztecs had been before them.
Although Cortés was a skilled leader, he and his force of perhaps a thousand Spaniards and indigenous allies would not have been able to overcome a city of 200,000 without help. He got it in the form of a smallpox epidemic that gradually spread inward from the coast of Mexico and decimated the densely populated city of Tenochtitlan in 1520, reducing its population by 40 percent in a single year.
Smallpox is caused by an inhaled virus, which causes fever, vomiting and a rash, soon covering the body with fluid-filled blisters. These turn into scabs which leave scars. Fatal in approximately one-third of cases, another third of those afflicted with the disease typically develop blindness.