A basic structural component is a model or models of disease progression. I say a model or models because the model of disease progression may differ for some subgroups. This is important because we need to model the stages of development of the disease in individuals that influence transmission. Thus the infectivity of infecteds generally changes as a disease progresses. In addition, the contact rates important for transmission may change as an illness progresses. Thus we need a model of disease progression for each population subgroup.