Evolution of increased susceptibility to infectious diseases in age structured populations

Current theory on the epidemiology of infectious diseases in genetically heterogeneous host populations ignores age structure. Thus, the possibility that microparasites might have negative effects on fitness in different ways during different phases of the lives of their hosts is not accounted for. For example, infections causing mortality and morbidity among juveniles might also have an impairing effect on reproduction among adults, as is the case in many so called childhood diseases. I demonstrate that when this is the case there may be a selective pressure for the host to evolve a higher level of susceptibility to an infection, provided it has the following properties: it should (i) provide life-long immunity, (ii) have a negative effect on reproduction and (iii) not cause too many mortalities.