The people did not have immunity against the disease.
The Inca and Aztec peoples were developed societies in the central and southern americas. These people lived only in their territory and did not have antibodies with the antigens present in the bodies of the Europeans. Thus, unintentionally, the white man transmitted the smallpox virus to these communities, which were practically decimated by the disease. When a society is isolated, such as the Incas and Aztecs, a seemingly harmless antigen can cause devastation, precisely because people still do not have antibodies to fight the disease.
