Migrating humans may have killed off Neanderthals by accident

[Scootaloo]

20. True, but that involved several factors that were not present in the paleolithic

Population density, for one. Ice Age Europe didn't have the sort of population and societal contacts to facilitate spread if diseases. Native America did.

For another, the existence of said diseases. You don't have smallpox until after you domesticate cattle. You don't get influenza until you have large flocks of poultry. You don't worry about bubonic plague without dense rat populations, which only come about with large agricultural stores. Typhus and cholera are diseases of dense populations, which only come from sedentary agricultural societies.