Multi-host pathogen transmission and the disease-diversity relationship

How diseases are transmitted within a multi-host community is a complex biological process with important ecological and societal consequences. The intricacies of interspecific disease transmission determine when a disease can spread to a novel host, including humans (zoonosis), and the severity of emerging epidemics. Interspecific disease transmission also mediates long-term disease prevalence within a multi-host community which is at the core of the disease-diversity relationship. Mathematical models play a central role in formulating predictions about spillover, prevalence, and the disease-diversity relationship. Yet, how the complexity of transmission is captured (or not) by the assumptions of these models is often unclear. Here, we decompose the transmission process into five biological stages using bovine tuberculosis (bTB) as an illustrative example of transmission in a multi-host system. We then examine the often-implicit assumptions that classic compartmental models make about this process. We use the intuition gained from this decomposition to formulate hypotheses for how transmission can mediate outbreak potential, infection prevalence, and the amplifying or diluting effects of host diversity on disease prevalence. We further illustrate the key principles and implications of transmission with a diverse array of examples of multi-host pathogens. Throughout we emphasise the role of evolution in shaping interspecific transmission, from the evolutionary relatedness of the hosts themselves to the adaptation of the pathogen to novel hosts.