The first step in the success or failure of emerging pathogens.

“In order to understand the dynamics of infectious disease outbreaks in humans, we really need to understand the dynamics of the pathogen in nature.”* Over the last decade, I have heard one particular gray-haired ecologist repeat this every chance he gets, for one good reason: It is true. Understanding how pathogens work in natural settings can inform us what consequences may lie ahead if a particular pathogen jumps from one species to another. Outbreaks start with the first or index case in a species. The pathology of the disease agent in that case will determine the fate of that pathogen in its new host species. Mollentze et al. (1) use a meta-analysis of infection challenge data to predict the likely significance, in terms of pathology, of a transspecies infection. This study has particular relevance for rabies and may highlight general implications for other infectious diseases jumping species. At the very least, beyond rabies, it provides an excellent framework for predicting transspecies pathologies for infectious diseases. Successful establishment of the index infection is reliant on complementary host and pathogen attributes. Pathogens have a finite, often streamlined, genetic code, which has evolved to take advantage by evading, attacking, or hijacking the host’s immune system. Coevolution hones the pathogen’s phenotype toward sustainable exploitation of its host’s population. When a pathogen jumps into a new host species, the outcome of that infection and its significance to the new host species is largely cryptic, ranging from benign to disastrous. Some of these outcomes can be predicted using laboratory infection studies across species. Moreover, larger studies or meta-analyses across studies can help us predict and assess risks involving transspecies infections (1). I will discuss some of the other aspects of pathogen biology, ecology, and evolution which precede and follow a pathogen jumping species. Pathogens … [↵][1]1Email: w.r.easterday{at}ibv.uio.no. [1]: #xref-corresp-1-1