'Small worlds' and the evolution of virulence: infection occurs locally and at a distance

Why are some diseases more virulent than others? Vector-borne diseases such as malaria and water-borne diseases such as cholera are generally more vir ulent than diseases spread by direct contagion. One factor that characteriz es both vector- and water-borne diseases is their ability to spread over lo ng distances, thus causing infection of susceptible individuals distant fro m the infected individual. Here we show that this ability of the pathogen t o infect distant individuals in a spatially structured host population lead s to the evolution of a more virulent pathogen. We use a lattice model in w hich reproduction is local but infection can vary between completely local to completely global. With completely global infection the evolutionarily s table strategy (ESS) is the same as in mean-field models while a lower viru lence is predicted as infection becomes more local. There is characteristic ally a period of relatively moderate increase in virulence followed by a mo re rapid rise with increasing proportions of global infection as we move be yond a 'critical connectivity'. In the light of recent work emphasizing the existence of 'small world' networks in human populations, our results sugg ests that if the world is getting 'smaller'-as populations become more conn ected-diseases may evolve higher virulence.