In the course of more than 300 million years of evolution, diverse and complex relationships have arisen among vertebrate species and their associated macro-and microbiota. Inevitably, there is considerable overlap among the families, genera, species and subtypes of organisms that have evolved to have commensal, parasitic, pathogenic and/or symbiotic relationships with vertebrate hosts. Absolute host specificity is the exception rather than the rule, and many organisms have some capacity to be transmitted among vertebrate hosts and to act, more or less frequently, as causal agents of disease (Viana et al., 2014). Some multi-host organisms, such as cestode parasites, have life cycles that are fully dependent on transmission among two or more host species. Others, such as Japanese B encephalitis virus, have harnessed the mobility of arthropods to expedite intra-and inter-species transmission. Bacteria such as Salmonella spp., Brucella spp. and Leptospira spp. have diverse host relationships, with individual serovars or biovars exhibiting varying degrees of host adaptation and pathogenic potential across multiple host species. At an ecological level, individual vertebrate species may serve a range of roles in the maintenance, transmission and emergence of multi-host pathogens in different ecosystems and socio-cultural contexts (Caron et al., 2014; Viana et al., 2014).