How did just a handful of Spaniards topple the mighty Aztec Empire?

Initial contacts between Europeans and natives on the North American continent always resulted in the death of a substantial percentage of the native population due to exposure to European diseases, especially Small Pox. Often by the time hostilities broke out between European invaders and natives, the native population had been so descimated they just didn't have a chance. I don't know the specific numbers for the Aztec Empire, but I can ure you that their numbers were not so large by the time hostilities broke out. Many historians today wonder what would have happened if Europeans had been fighting a larger native population.