Was it smallpox that killed so many Aztecs?

An alternative alternate theory says that instead of cholera it was some native respiratory disease. The Aztecs were all crowded in one place, and got hit by a mutated form of an epidemic disease they’d had around for a few centuries.

The first hand account The Conquest of New Spain by Bernal Diaz is remarkably accessible.

Whichever pathogen(s) spread havoc, it’s clear it started where the Spanish did and moved with them.

What’s also interesting is how little effect guns had on the Aztec. The first major battle where the Atzecs confronted muskets the Aztecs looked around confused after the first “bang,” and so the Spaniards where able to clear the field with no great loss to either side. However, in subesquent battles the Atzecs had learned that to once a gun went “booom” it was harmless, so the Aztecs learned to play a “cat-and-mouse” game to get the musketeer to discharge and then run in and butcher him. What did (militarily) make a difference were the steel breast plates that allowed the Spaniards to fight in close-ordered infantry formations. The leather-wearing jade wielding aztecs could only replicate these tactics at frightening casuality levels. Thus, the Spaniards, like a modern American armored unit, could seize any particular bit of the ground they wanted but typically could not “clear the field” of their numerically superiors opponents unaided. Hence the Spaniard’s strategy of having lots of Indian allies with them (to cover the flanks and sweep up broken Aztec units) while focusing on capturing or killing the leaders of whatever force opposed them.

..but yeah, the pathogens were what brought the Aztecs low, as well as Cortes’s own allies.