Quote from the internet:
"Human evolution known to be a process in which species adapt to certain conditions of life, and in the battle of life and death, they are forced to become stronger in order to survive in this world. Diseases have always been present during the evolution, acting themselves as a natural selectors - the weaker organisms get sick and die. Not just humans, but all species have to establish a great protecting mechanism in order to survive. That is how our bodies are built, to be able to withstand and survive in the conditions we live in, and that is why our immune system has to be taken care of. "
Basically this is saying that through evolution and natural selection, a population will be able to defend itself against a common disease.
Example: those who are heterozygous for sickle cell anemia are immune to malaria, a disease that is common in places like west Africa and has is very deadly
