which reason explains why diseases like smallpox had such a devastating effect on the aztec and inca populations

The answer is in the description

Explanation:

The Americans  (native) didn`t have the immunities that the Europeans had. Imagine how many died in the black plague. Afterwards, the survivors became immune, the Natives never got black plague. So when they DID get it, there bodies couldn`t handle the black death AND the other diseases carried by the Europeans

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i think it is true i hope this is

the answer would be a).