Invasive disease caused by Haemophilus influenzae type b (Hib) is typically a disease of young children, starting when the protection afforded by maternal antibodies is exhausted and ending when the child's own antibody production to the Hib capsular polysaccharide begins efficiently. There are, however, large variations between population groups in the incidence of the disease and the age at which it appears. Potential factors behind this variation include virulence genes of the bacteria and genetic susceptibility genes of the host. Although such genes and their effects can be identified, their impact on the disease's incidence is only minor. Potentially much more important appear to be socioeconomic factors that to a large extent determine the exposure of the child to Hib and to other infectious agents that may enhance both colonization and invasive infection by Hib.