What does the relationship between genetic traits and the prevalence of diseases such as malaria and smallpox illustrate?

The relationship between genetic traits and the prevalence of diseases such as malaria and smallpox illustrates the ways in which human biological diversity reflects adaptation to such environmental stresses as disease, diet, and climate.
 Positive selection increases the prevalence of genetic variants that improve survival and fertility.

The relationship  between genetic traits and the prevalence  of diseases such as malaria  and smallpox illustrate, the  way human  biological diversity  reflects the adaptation  to environmental  stresses such  as  diseases , diet and climate.

For example  high   prevalence of  blood  group O studies  example in country like india  show  that blood group  O has a less sever malaria  as compared  to  blood group A, B  andAB.  This  suggested that  individual with   blood  O  who live  within area   which is prone to malaria  have high  resistance to malaria