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

Natural selection, is based upon the principle of "Survival of The Fittest" ensures that those who are more well adapted, especially having a better immune system, will survive. and the weaker ones will die out. Human evolution made the human immune system even better as time went by, and the system was more well adapted to diseases it was more exposed to like the Europeans were more resilient to smallpox than the native americans