Experimental evidence for stabilising selection on virulence in a bacterial pathogen

The virulence-transmission trade-off hypothesis has provided a dominant theoretical basis for predicting pathogen virulence evolution, but empirical tests are rare, particularly at pathogen emergence. We conducted an infection experiment in a North American songbird using 55 distinct isolates of its emerging infectious bacterial pathogen that differed in virulence. We demonstrate that more virulent variants transmitted faster, but had shorter infection durations, leading to variants of intermediate virulence being more evolutionarily successful. Interestingly, we did not find support for the common suggestion that the number of pathogen cells underpins their virulence and transmission rate.Â