Every “infective disease” (the term is Garrod’ s) has two components: the susceptibility of the host and the virulence of the pathogen. Infection is a contest between two genomes, expressed in two competing organisms. This chapter addresses various examples of host susceptibility and resistance to infectious agents, illustrated by examples such as the hemoglobin variants, G6PD deficiency, malaria, mycobacterial diseases, HIV and AIDS, and hepatitis.