How did disease such as smallpox play role in the fall of the Aztec empire

Aztec empire isolated themselves from other nations and they don't know how to cure diseases like smallpox and most of the people died from the smallpox brought by the Spanish

When the Spanish attacked the Aztecs, they brought disease with them. The Spanish started to build a resistance because their bodies were used to it. The Aztecs haven't been exposed to anything like Smallpox so they had no immunity weakening their army, making people fear them.