Patients suffering from a serious illness frequently ask “Why did this happen to me?” When the disease is cancer or cardiovascular disease, patients recognise the risk of inheriting “bad genes” from parents as readily as the risks from smoking and diet. It is all too clear that if both our parents suffered myocardial infarcts at an early age we must be at increased risk of the same thing happening to ourselves. However, when asked why someone developed a serious infection, we generally blame lack of acquired immunity, environmental factors, or bad luck. Increasingly it appears that “bad luck” really means the genes we have inherited.

It is a common misapprehension that our genes are not important in determining our ability to fight off infectious diseases. In fact a study of almost 1,000 adoptees in Denmark found that the host genetic component of susceptibility to premature death from infection is greater than for cancer and cardiovascular disease (Sorensen et al., 1988). This is not unexpected as common diseases which cause high mortality exert the greatest evolutionary effects on the human genome. Prior to this century infectious diseases were the major cause of death in the western world and still are in many developing countries. From this we can surmise that microorganisms have been the major selective force in recent human evolution. In other words the interaction between the genes of our ancestors and those of human pathogens have resulted in what makes each of us genetically unique today.