The results of this study formed the basis of a general model of the evolutionary process which is called directionality theory. We now apply this theory to analyse the effect of finite size in multi-strain epidemiological models where the heterogeneity derives from variability in infection age. The quantity V (x) in this class of model pertains to the product of the survivorship and infectivity of infectious individuals. We will apply the formalism described in [19] [20] [21] [22] to show that the invasion dynamics of competing strains in populations of finite size is predicted in terms of the macroscopic variable evolutionary entropy, S, which is given by