Almost all diseases result from a complex interaction between an individual’s genetic make-up and environmental agents. Subtle differences in genetic factors cause people to respond differently to the same environmental exposure. This explains why some individuals have a fairly low risk of developing a disease as a result of an environmental insult, while others are much more vulnerable.
From an evolutionary perspective, infectious diseases have probably been the primary agent of natural selection over the past 5000 years, eliminating human hosts who were more susceptible to disease and sparing those who were more resistant. Individual biological factors that conferred protection from a specific disease could eventually be selected for in the human populations living where the disease was endemic. Thus, as first suggested by Haldane (1949), humans adapted to infectious disease at a most basic level, the level of the gene.
From Inhorn and Brown (1990)