Fig. 3:

The empirically-observed effects of incomplete immunity result in an almost twofold increase in optimal virulence relative to a model with no immunity (A). Pathogen fitness is measured as invasion success, computed from the proportion of resident pathogen parameter space over which an invader of a given virulence level was able to successfully displace the resident pathogen in the PIPs for each model (B,C), scaled to a maximum invasion value of 1 (see supplementary materials and (35)). Shaded areas in the PIPs show parameter space for which a new mutant introduced at very low densities was able to invade a population with equilibrium densities of the resident strain. Non-shaded areas indicate parameter space where the resident strain cannot be competitively displaced, and the asterisks in the center of this space thus mark the Evolutionary Stable Strategy (ESS) for each model. Susceptibility η and mortality υ were both continuous functions of strain virulence (ε, Fig. 2); all other parameters were held constant (eqns. S2).