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Significance
Emerging pathogens are usually far from any phenotypical optimum in relation to their new hosts. The concept of evolutionarily stable strategies (ESS) may be helpful for predicting the evolution of such pathogens. The ESS represents a pathogen variant with an optimal phenotype, which cannot be outcompeted by invading mutants. We investigate the ESS for a pathogen in an age-structured population, assuming a trade-off between pathogen infectivity and the duration of disease analogous to the classic virulence–infectivity trade-off. We show how a population divided into age stages with different demographic parameters affects the evolution of the pathogen and shapes its traits. As many human pathogens cause a highly age-dependent disease burden, this provides important projections about the direction of evolution. Source: pnas.org