Stepping-stones and Mediators of Pandemic Expansion—A Context for Humans as Ecological Super-spreaders

Humans represent ecological super-spreaders in the dissemination and introduction of pathogens. These processes, consistent with the dynamics of the Stockholm paradigm, are exemplified in the origin and globalized distributions of SARS-CoV-2 since initial recognition in central Asia during 2019 and 2020. SARS-like viruses are not widespread in mammals but appear widespread in chiropterans. Bats are isolated ecologically from most other assemblages of mammals in terrestrial systems. Humans may be the stepping-stone hosts for broad global dissemination and wider infection (given the opportunity) among diverse assemblages of mammals in which host and viral capacity are compatible. Human globalization mediated insertion in global ecosystems along primary and secondary pathways initially with localized to regional circulation across continents. Origins and persistence of cycles involving variants and viral transmission among other mammals and the potential for secondary exposure (retrocolonization) of people occurs on multidirectional pathways. Humans were responsible for the initial breakdown in ecological isolation of the virus that facilitated colonization events from chiropterans to other mammals. In the absence of these human drivers, environmental or ecological interfaces (boundary zones among habitats) limiting the distribution of SARS-CoV-2 are unlikely to have been crossed, leaving a diverse assemblage of SARS-like viruses (Sarbecovirus) remaining relatively hidden and isolated in southeast Asia.