1519 (March 4): The Spaniards land in what is now Veracruz. The natives greet him with gifts. A contingent from Tenochtitlán also arrives with gifts. Cortes showed force and demonstrated his canons, terrifying the messengers.
Cortes sinks all the ships except one small vessel, and moves to the city-state of Tlaxcala. Generally speaking they begin on friendly terms.
The Spanish went on to Cholula. It's unclear what happened, but in the end many people in the city were killed by the Spanish.
1519 (Nov 8): Cortes and army arrives at Tenochtitlán, and begin on good terms. However, Cortes takes charge and demands tribute and some Roman Catholic shrines to replace the Aztec gods.
Cortes returns to the coast to defeat a rival Spanish army. The remaining soldiers join him.
The Aztecs revolt, driving the Spanish from the city. Cuitláhuac becomes emperor.
The Spanish go to Tlaxcala and make an alliance with them.
Most Aztec cities are conquered, and a siege of Tenochtitlán begins.
1521 (13 August): The last Aztec emperor, Cuauhtémoc, surrenders to Cortes.
Answer: Malaria inflicts great casualties and affects entire army's indiscriminately. Soldiers were exposed and vulnerable.
Explanation:
It is believed that Alexander the Great was killed by malaria at the height of his power
Malaria in Afghanistan, Iraq, and Liberia (2001–2003): Many US soldiers in Iraq walked while eating just to avoid being bitten and infected by mosquitoes.
Vietnam War (1962–1975): Malaria felled more combatants during the war than bullets. The disease reduced the combat strength of some units by half. Over 40,000 cases of Malaria were reported in US Army troops alone between 1965 and 70 with 78 deaths. The U.S. Army established a malaria drug research program when U.S. troops first encountered drug resistant malaria during the war
During the American civil war in 1861-1865, malaria accounted for 1,316,000 episodes of illness and 10,000 deaths. It has been estimated that 50% of the white soldiers and 80% of the black soldiers got malaria annually.
Source: https://www.malariasite.com/wars-victims/
