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

Human evolution, and natural selection are processes during which humans/living beings get accustomed to their surroundings, improve, and fight for survival, because if they don't, they will die. Diseases have always been present during the evolution and natural selection processes, as a way of determining who is fit enough to survive, and who will succumb to those diseases and disappear. 

Of the cold they can survive in the cold