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

Human evolution and natural selection both greatly relate to the susceptibility of a disease. For every disease that passes through, there will likely be a small handful of people who happen to be immune.

For example, the Black Death was a plague that peaked in Europe around year 1350. This disease unfortunately killed hundreds of people, but there were the select few who were immune. Later, these people had children and passed the trait down. The trait has likely continued to be passed down to the point where a majority of the population is likely immune to this particular disease. 
This is an example of both human evolution and natural selection. Humans have evolved and passed this "immunity" trait down to children and grandchildren, and now it is unlikely that many people will be affected if this disease happens to pass through again. 
This is also an example of natural selection because the people who were susceptible to the disease likely caught it and passed away. These were the people who were not considered "fit" for the environment and the disease.

Human evolution and natural selection work with each other to evolve humanity and our susceptibility to disease. Unfortunately, it is beyond likely that new diseases will constantly arise and may harm humans. However, as we continue to evolve, we are much less likely to catch previous diseases.