Figure 1.
Trade-off models for virulence evolution.
(A) A typical trade-off curve for virulence evolution, in this case between transmission rate and host mortality. The trade-off curve is a boundary on the mortality rate and transmission rate that the parasite can evolve. If the characteristics of the pathogen initially lie underneath the trade-off curve (red circle), the early evolution will be toward the boundary, and then along it, shown by arrows. (B) Parasite fitness (R0) is proportional to the ratio of transmission rate over the sum of recovery rate and mortality rate. By displacing the trade-off curve along the horizontal axis by an amount equal to the rate of recovery, the R0 of any point on the trade-off curve is simply the slope of the line from the origin to the point. The maximum R0 is thus achieved at the tangent of a line through the origin, as shown. This figure illustrates how the choice of the trade-off function affects what can be concluded about the evolution of virulence (mortality). Although recovery rate affects the optimum parasite fitness (R0), recovery is unaffected by evolution when it is not part of the trade-off (as shown here). However, if the trade-off instead was between transmission and recovery (swapping recovery and mortality rates on the x-axis), evolution of parasite fitness would affect only the rate of host recovery, and there would be no predictions about host mortality. (C) Data on R0 and pre-vaccination mortality rates for the viruses indicated (bottom) are abstracted from [16], [17] and references therein. VZV, Varicella zoster virus.