Immunity-driven evolution of virulence and diversity in respiratory diseases.

Abstract

The time-honoured paradigm in the theory of virulence evolution assumes a positive relation between infectivity and harmfulness. However, the aetiology of respiratory diseases yields a negative relation, with diseases of the lower respiratory tract being less infective and more harmful. We explore the evolutionary consequences in a simple model incorporating cross-immunity between disease strains that diminishes with their distance in the respiratory tract, assuming that docking rate follows the match between the local mix of cell surface types and the pathogen's surface, and cross-immunity the similarity of the pathogens' surfaces. The assumed relation between fitness components causes virulent strains infecting the lower airways to evolve to milder, more transmissible variants. Limited cross-immunity generally causes a readiness to diversify that increases with host population density. In respiratory diseases, that diversity will be highest in the upper respiratory tract. More tentatively, emerging respiratory diseases are likely to start low and virulent, to evolve up and become milder. Our results extend to a panoply of realistic generalisations of the disease's ecology and to including additional epitope axes. These extensions allow us to apply our results quantitatively to elucidate the differences in diversification between rhino- and coronavirus caused common colds.