An epidemiological context for the consequences of phenotypic plasticity in host-pathogen interactions

Questions: What effect do different forms of strategic plasticity have on the co-evolution of host and pathogen? We focus on the co-evolution of pathogen exploitation strategies (virulence) and the rate at which the host immune system clears the pathogen (clearance rate). Mathematical methods: Evolutionary game theory; computer simulations of host-pathogen pairs negotiating strategies using linear response rules. Key assumptions: A trade-off exists between virulence and transmission rate for pathogens, and between fecundity while infected and recovery (clearance) rate for hosts. Disease dynamics are described by a standard susceptible-infected-susceptible epidemiological model. Transmission of the pathogen is exclusively horizontal, and random mixing of the host population is assumed. Conclusions: All forms of plasticity promote the co-evolution of virulence and clearance rates that are lower than those predicted in the absence of plasticity. Plasticity promotes increased disease incidence rate (higher than those predicted in the absence of plasticity), but the way it affects case mortality depends critically on the assumed mode of plasticity.