One of the most significant parameters in epidemiology is the duration of host infection, D, which is taken to represent the period of time for which an infective is capable of transmission to susceptibles. This parameter is related to T, the generation time. D can be generally expressed in the form D~kT where k is a parameter dependent on the strain, but the characterisation of D will depend on the model system under consideration. In a basic S-I-R model, for example, the fecundity function is a constant independent of infectious age. Consequently the generation time will depend uniquely on the mortality rate (i.e. the rate of recovery from the pathogen). The generation time T will be inversely related to the rate at which individuals leave the infected class, denoted by n. Hence for this class of models D~kT~1=n. In the model described in this paper, survivorship and fecundity are functions of age, so the generation time involves both survivorship and fecundity components. The generation time T can be expressed in terms of the entropy function S. As shown by Demetrius et al. [31] , the evolutionary entropy can be shown to be analytically related to T by This equation asserts, therefore, that in systems with demographic heterogeneity the duration of infection will now be regulated by the pathogen entropy, S. This important fact underscores the fact that demographic heterogeneity dictated by V (x) will induce significant changes in the epidemiological dynamics.