Variation in host resistance could limit the spread of more broadly virulent pathogens

One major question in studies on the ecology and evolution of infectious diseases is whether enhanced host resistance will drive the evolution of more virulent parasites. To date, theory, laboratory studies, and field studies have all shown an association between increased host resistance and higher frequencies of virulent strains, suggesting that enhanced host immunity could have the undesirable side effect of favoring virulent pathogen genotypes during co-infection. There is also evidence, however, that selection through susceptible hosts could lead to pathogen genotypes that are more capable of infecting and causing disease in highly resistant host genotypes. What could account for the difference in results? In all of the vertebrate host–pathogen experiments to date, evolved pathogen virulence has been evaluated in the same host genotype as the evolution took place, and so one explanation is that the evolution of more broadly virulent pathogens in a vertebrate host–pathogen system could be limited by pathogen fitness trade-offs during infection of hosts with different levels of genotypic resistance. In this issue of Virulence, Kubinak and Potts present an elegant set of experiments which test and support this hypothesis. The gold standard for empirically studying pathogen adaptation is serial passage, which involves repeated transmission of a pathogen through a succession of hosts. Recently, Kubinak et al. serially passaged Friend virus (FV) through a single strain of inbred congenic mice to test how differences at MHC (major histocompatibility complex) loci could influence patterns of viral adaptation and Variation in host resistance could limit the spread of more broadly virulent pathogens