The reasons why some flu strains are more pathogenic than others is a complex issue involving specific details of host-virus interactions, but the model we propose has the attraction of capturing (on a simple criterion of pathogenicity, at least) evolution to generally less-pathogenic strains. The determinants that drive empirically observed patterns of emergence and spread of novel infectious pathogens are incompletely understood, turning as it does on the interplay of epidemiological, immunological and genetic considerations. No single model is able to capture the full complexity of this reality, but the work presented here is intended to shed some light on the criteria for invasion success and subsequent evolution of emergent strains.