What events led to the decline of Aztec empire?

The major event was the invasion of the Spanish conquistador Hernán Cortés (aka Hernando Cortez 1485-1547), who tricked the Aztecs into them worshipping him, and then attacked with his superior military and destroyed them.

There was also the exposure to European diseases and the lack of national unity, as many of the people of the region disliked the rule of the Aztecs (under Moctezuma II). These factors also led to the decline of the Incas in Peru.

The Effect of An Indigenous Disease

The "Cocolitzli" sickness ultimately caused the demise of the Aztec society. Cocolitzli appeared immediately after long, severe periods of geographic drought. Contemporary research suggests a mutated variation of the Hantavirus, carried by rodents ultimately caused the Cocolitzli which in essence wiped out 80% of the indigenous populations. In fact, the disease ravaged Spanish clergy as well, who had a high degree of exposure to native populations, by virtue of their close association with the peoples as part of the conversion process. Contemporary research suggests European disease had limited impact on decline of the Aztec empire.

After periods of drought, followed immediately by a torrentially wet year, rodent populations exploded because of the newly abundant sources of food. This created huge numbers of Hanta carriers, exposing the human population to the virus. Research also suggests that the Hantavirus mutated, and was subsequently able to jump the species barrier-from human to human, similar to that of the "Swineflu (H1N1)" virus and the "Birdflu" virus. The tenacity of the virus caused the rapid spread from person to person, decimating the population in the process. Drought in the New Mexico, Arizona, Utah, and the Colorado area caused a spat of Hanta in 1993 which caused the deaths of 58 Americans before the CDC was able to isolate the rodents and contain the virus through culling. Forensic disease researchers believe the same environmental factors in 1993 which caused the Hanta outbreak also were responsible for the Aztec Cocolitzli and conquest of the culture.

Certainly the musket and the cannon played a part in the demise of native populations, though ultimately it was disease that decimated the Aztecs. It wasn't smallpox, nor measles, mumps, rubella, or plague. But a highly dangerous and virulent strain of the Hantavirus- a bug which is as dangerous today as it was 500 years ago.