Dissecting transmission to understand parasite evolution

The prevailing theory of the evolution of virulence assumes that evolution maximizes its transmission and relies on a trade-off between virulence and the parasite's transmission rate. While this simple idea finds some empirical support, it is often criticized, in part because of its ambiguity about transmission, the key measure of pathogen fitness. In theoretical and empirical studies, transmission has been increasingly approximated by parasite load. Transmission, however, is a complex parameter that results from distinct steps within and among hosts, with potential correlations and trade-offs among each of the steps. We propose that decomposing explicitly transmission into these steps would enable more precise predictions and a deeper understanding of parasite transmission dynamics and virulence evolution.