Disease was the primary cause of depopulation in all of the new world, it was not less devastating in areas south of the modern US. In fact it was probably more devastating as the native populations initially encountered by the Spanish in Mexico were larger and more concentrated than ones to the north, therefore the spread of disease was easier. Compound that with famine that occurred after the conquest of Mexico, populations were devastated.

In the early 1500s the Aztec population was estimated to be around 25 million, and with in a hundred years the population had dropped to less than 2 million. That's an over 90% reduction in population. I'm not sure what gave you the impression that Native populations farther north faced more devastation to their population.

hi, sounds like you're asking more about current population demographics, than impact of disease per se. If so, you can get started in the FAQ:

• Racial differences between North America and Latin America