Does multiple infection select for raised virulence?

Classical models of virulence evolution conclude that the increased competition favoured by multiple infection will select for increasing consumption and deterioration of the host resource, or 'virulence'. However, recent empirical and theoretical studies suggest that this view of virulence has some shortcomings. Here, we argue that the evolutionary consequences of multiple infection depend critically on whether the exploitation rate of an individual parasite is governed directly by the behaviour of the individual, or whether it is limited by the collective behaviour of the coinfecting group. We illustrate that, depending on the mechanistic details of exploitation, multiple infection can select for reduced virulence.