Pathogens pull hardest in the coevolutionary arms-race to determine age-specific transmission biases.

Juveniles across a wide range of species are more susceptible to pathogenic infection than their adult conspecifics, often driving epidemic spread. While high juvenile susceptibility can be related to developmental timing, recent theoretical studies have found that hosts often evolve high juvenile susceptibility given a trade-off with other host traits. However, most research on age-specific infection has focused solely on host evolutionary dynamics, and the effect of pathogen evolution or coevolution on juvenile infection rates remains unknown. Here, we developed a coevolutionary adaptive dynamics model to investigate age-specific infectivity and contrast the outcome of coevolution with host evolution and pathogen evolution alone. We found strong differences in the evolutionary dynamics of hosts and pathogens in the absence of coevolution. While host evolution often favoured genotypes with intermediate levels of juvenile and adult resistance, pathogen evolution selected for extreme specialists. When hosts and pathogens were allowed to coevolve, the dynamics more closely resembled the pathogen dynamics; pathogens rapidly evolved to specialize on infecting a specific age class and this drove reciprocal specialization in host resistance. Our results illustrate that coevolution has the potential to shape age-specific patterns of infection that impact disease spread.