The Role of Punctuated Evolution in the Pathogenicity of Influenza Viruses

Abstract

ABSTRACT Influenza is an acute respiratory disease caused by influenza viruses. Evolutionarily, all influenza viruses are zoonoses, arising in the animal reservoir and spilling over into the human population. Several times a century, one of these zoonotic events results in a new influenza virus lineage becoming established in humans and circulating for years or decades as an endemic strain. The worldwide pandemic that occurs shortly after the nascent virus becomes established can have a profound impact on morbidity and mortality. Because influenza viruses continually evolve and the illness they engender can vary considerably based on characteristics of the strain, the weather, other circulating or endemic pathogens, as well as the number of susceptible hosts, the impact of each season on human health is unpredictable. Over time, the general pattern is for pandemic strains to adapt and gradually take on characteristics of seasonal strains with lower virulence and a diminished synergism with bacterial pathogens. Study of this punctuated evolution yields a number of insights into the overall pathogenicity of influenza viruses.