There is a rich literature highlighting that pathogens are generally; better adapted to infect local than novel hosts, and a separate; seemingly contradictory literature indicating that novel pathogens pose; the greatest threat to biodiversity and public health. Here, using; Batrachochytrium dendrobatidis, the fungus associated with; worldwide amphibian declines, we test the hypothesis that there is; enough variance in novel host-pathogen outcomes to pose substantial risk; of pathogen introductions despite local adaptation being common. Our; continental-scale, common garden experiment and global-scale; meta-analysis demonstrate that local amphibian-fungal interactions; result in higher pathogen prevalence, pathogen growth, and host; mortality, but novel interactions led to strikingly variable; consequences with the greatest risk occurring when susceptible hosts and; virulent strains interacted. Thus, while most pathogen introductions are; benign, enough variance exists in novel host-pathogen outcomes that; moving organisms around the planet greatly increases the chance of; pathogen introductions causing profound harm.
