Microbial Virulence as an Emergent Property: Consequences and Opportunities

Although an existential threat from the microbial world might seem like science fiction, a catastrophic decline in amphibian populations with the extinction of dozens of species has been attributed to a chytrid fungus [1], [2], and North American bats are being decimated by Geomyces destructans, a new fungal pathogen [3]. Hence, individual microbes can cause the extinction of a species. In the foregoing instances, neither fungus had a known relationship with the threatened species; there was neither selection pressure for pathogen attenuation nor effective host defense. Humans are also constantly confronted by new microbial threats as witnessed by the appearance of HIV, SARS coronavirus, and the latest influenza pandemic. While some microbial threats seem to be frequently emerging or re-emerging, others seem to wane or attenuate with time, as exemplified by the decline of rheumatic heart disease [4], the evolution of syphilis from a fulminant to a chronic disease [5], and the disappearance of “English sweating sickness” [6]. A defining feature of infectious diseases is changeability, with change being a function of microbial, host, environmental, and societal changes that together translate into changes in the outcome of a host–microbe interaction. Given that species as varied as amphibians and bats can be threatened with extinction by microbes, the development of predictive tools for identifying microbial threats is both desirable and important.