In the New World, why did diseases such a smallpox have such a devastating effect on Native Americans but only a minor effect on Europeans.

The reason was very simple. Smallpox, for example, has an incubation period of 10 to 14 days, and the virus is spread by respiration and also by contaminated personal clothing and bedding. When the first symptoms appeared in an aboriginal settlement, individuals who had not yet developed the disease had moved elsewhere, carrying with them the infection. In other words, the diseases spread faster than the conquerors themselves.

Smallpox was unknown among the indigenous people, they had never had contact with them and therefore had not had the possibility of developing immunity against them. That is why diseases that were relatively benign among Europeans (who had already developed antibodies) had devastating effects on the indigenous population.