Europeans exploring and settling North America in the 16th and 17th centuries brought smallpox, measles and influenza, diseases usually associated with domesticated livestock. Native Americans, who generally were in better health than Europeans, did not keep livestock and had no immunity to these diseases.
They were were exposed to many diseases their immune systems were not yet prepared to cope with, such as smallpoxs, influensa (the flu), and measles. It also introduced new social patterns and opportunities for trade, as well a introduced them to animals not native to the americas. (these livestock were a large contributor to the introducing of the diseases previously mentioned.)
Conquered territories were often left relatively unchanged under their old rulers as long as they recognized Aztec supremacy and paid tribute. Hope this helps
