Little Appleton Pasteuria epidemic dataset

Virulence, the degree to which a pathogen harms its host, is an important but poorly understood aspect of host-pathogen interactions. However it is not a static trait, instead depending on ecological context and potentially evolving over short periods of time (e.g., during the course of an epidemic). At the start of an epidemic, when susceptible hosts are plentiful, pathogens may evolve increased virulence, maximizing their intrinsic growth rate. However, if host density declines during an epidemic, theory predicts evolution of reduced virulence. Although well-studied theoretically, there is still little empirical evidence for virulence evolution in epidemics, especially in natural settings with native host and pathogen species. Here, we used a combination of field observations and lab experiments in the Daphnia-Pasteuria model system to look for evidence of virulence evolution in nature. Controlling for environmental conditions, we found that there was no change in parasite virulence w...