Expanding host specificity and pathogen sharing beyond viruses.

Most emerging pathogens of humans can infect multiple host species (Woolhouse & Gowtage-Sequeria, 2005). This simple fact has motivated multiple large-scale, comparative analyses of the drivers of pathogen sharing and zoonotic pathogen richness among hosts as well as the factors determining the zoonotic potential of pathogens themselves. However, most of this work focuses on viruses, limiting a broader understanding of how host range varies within and between pathogen groups. In this issue of Molecular Ecology, Shaw et al. (2020) compile a comprehensive data set of host-pathogen associations across viruses and bacteria and test whether previous patterns observed in the former occur in the latter. They find most viruses and bacteria are specialists, and viruses are more likely to be generalists; however, generalist bacteria encompass multiple host orders, whereas viral sharing occurs more within host orders. Lastly, the authors demonstrate that many factors previously identified as predictors of zoonotic richness for viruses occur for bacteria and that host phylogenetic similarity is a primary determinant of cross-species transmission. However, pathogen sharing with humans was more common and more weakly related to phylogenetic distance to Homo sapiens for bacteria compared to viruses, suggesting the former could pose greater spillover risks across host orders. This work represents a key advance in our understanding of host specificity and pathogen sharing beyond viruses.