Srijan Seal, Guha Dharmarajan, imroze khan

Researchers worldwide are repeatedly warning us against future zoonotic diseases resulting from mankind’s insurgence into natural ecosystems. The same zoonotic pathogens that cause severe infections in a human host fail to produce any disease outcome in their natural hosts. What precise features of the immune system enable natural reservoirs to carry these pathogens so efficiently? To understand these effects, we analyse the evolutionary basis of pathogen tolerance in reservoir hosts, while drawing implications from their diverse physiological and life-history traits, and ecological contexts of host-pathogen interactions. Long-term co-evolution might allow reservoir hosts to modulate immunity and evolve tolerance to zoonotic pathogens, increasing their circulation and infectious period. Such processes can also create a genetically diverse pathogen pool by allowing more mutations and genetic exchanges between circulating strains, thereby harbouring rare alive-on-arrival variants with extended infectivity to new hosts (i.e., spillover). Finally, we end by underscoring the indispensability of a large multi-disciplinary empirical framework to explore the proposed link between evolved tolerance, pathogen prevalence and spillover in the wild.