This is Atahualpa the INCA were not resistant to smallpox. When Pizzarro came he gave smallpox to Peru. Atahualpa army was way behind technology wise, they had bronze weapons and the conquistadors had metal and steel. Atahualpa people were dying before they came. The Inca were so blindsided by them looking like gods that they didn't even notice how dangerous they were or how contagious. Many Inca died from smallpox.
Smallpox hasn't real impact America because it's not around anymore. For a while people has to get smallpox vaccine.
Images from image quest