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

Natural selection is a one of the basic mechanisms of evolution by which species adapt to their environment in order to increase their chance of survival and reproduction. Diseases can act as natural selectors. That means that the weaker organism will get sick and die. Stronger will survive and have the ability to pass down their trait to the next generation. This suggests that species must have a great protecting mechanism in order to survive and reproduce.