Within-host priority effects and epidemic timing determine outbreak severity in coinfected populations

Coinfections of hosts by multiple pathogen species are ubiquitous, but predicting their impact on disease remains challenging. Interactions between coinfecting pathogens within hosts can alter pathogen transmission, with the impact on transmission typically dependent on the relative arrival order of pathogens within hosts (within-host priority effects). However, it is unclear how these within-host priority effects influence multi-pathogen epidemics, particularly when the arrival order of pathogens at the host population scale varies. Here we combined models and experiments with zooplankton and their naturally co-occurring fungal and bacterial pathogens to examine how within-host priority effects influence multi-pathogen epidemics. Epidemiological models parameterized with within-host priority effects measured at the single host scale predicted that advancing the start date of bacterial epidemics relative to fungal epidemics would decrease mean bacterial prevalence in a multi-pathogen se...