“There have been many deadly pandemics that have had severe effects on human populations such as malaria, cholera, plague, and HIV. Some human genetic variants are associated with protection against these diseases, whereas others are associated with vulnerability to them. Because they have had such an influence on a population level, these pandemics have influenced human genetic variation through evolution.
Protective alleles are more common in areas that have been ravaged by certain diseases than in other areas. Yet, not all beneficial alleles have reached fixation in the population and not all vulnerability alleles have been eliminated. Basic evolutionary considerations can explain these observations.”