Host–Pathogen Coevolution: Emergence of Microbial Virulence in a Changing Global Environment

Host–pathogen interactions represent dynamic evolutionary systems in which hosts continuously develop mechanisms to resist infection while pathogens evolve strategies to colonize, evade immunity, exploit host resources, and transmit successfully. This reciprocal process, commonly described as host–pathogen coevolution, is a major driver of microbial adaptation, pathogen emergence, and variation in disease severity. Virulence is not a fixed property of a microorganism but an evolving phenotype shaped by interactions among pathogen genetics, host susceptibility, ecological conditions, transmission routes, and environmental pressures. Contemporary global environmental change is increasingly modifying these interactions. Rising temperatures, altered precipitation, extreme weather events, habitat fragmentation, biodiversity loss, urbanization, agricultural intensification, pollution, and increased movement of people and animals can change host distributions, pathogen survival, transmission opportunities, and selective pressures. Climate-related environmental changes may therefore influence not only where pathogens occur but also how they evolve. Recent research emphasizes that the effects of climate change on infectious disease are highly pathogen- and context-specific and may involve nonlinear interactions between environmental conditions, host physiology, vectors, microbiomes, and pathogen evolution. Climate extremes can additionally disrupt plant microbiomes and host immunity, increasing disease risks in agricultural ecosystems. At the molecular level, mutation, recombination, horizontal gene transfer, mobile genetic elements, immune selection, and within-host competition contribute to the emergence of novel microbial phenotypes. Antimicrobial resistance further illustrates how environmental and anthropogenic pressures can accelerate pathogen evolution. Understanding these processes requires integration of evolutionary biology, microbiology, ecology, genomics, epidemiology, and climate science.