Incomplete host immunity favors the evolution of virulence in an emergent pathogen

Ratcheting up wild virulence Partially protective vaccination can sometimes select for increasingly virulent pathogens. Fleming-Davieset al.asked what happens in a natural system. In the United States, the house finch population is suffering an increasingly virulent epidemic caused byMycoplasma gallisepticum. The pathogen induces incomplete immunity that clears less virulent pathogens and offers partial protection against strains of greater virulence. In the birds, the partial immune response does away with competition from the less virulent pathogens. The partial immunity of the host also hinders replication of the more virulent pathogens enough to allow some birds to survive. This allows increasingly virulent forms of the pathogen to be transmitted. Science, this issue p. 1030