How does human evolution or natural selection relate to the susceptibility of disease?

Human evolution or natural selection means that our ancestors of up  to 3 million years ago in their various strains of say australopithecus or even Neanderthal man would be selected to flourish if they could resist disease better than say other strains of homo sapiens. An example of a case where foreign disease wss introduced was when the Spanish conquistadores came to Peru they brought smallpox which decimated the population or in British Columbia it was deliberately introduced to remove resistance to gold mining in the BC interior. The Incas and the BC First Nations simply had no immunity to the disease having had no contact with it before.