The implications of host-pathogen co-evolutionary outcomes on macro-epidemics based on a combined-host strategy.

Host defense and pathogen virulence interact and mutually shape each other's evolution. Host-pathogen co-evolutionary outcomes have potentially significant impacts on population dynamics and vice versa. To investigate host-pathogen interactions and explore the impact of micro-level co-evolutionary outcomes on macro-level epidemics, we develop a co-evolutionary model with a combined host-defense strategy. Our results illustrate that host-pathogen co-evolution may induce infection cycling and lead to the vanishing of the disease-induced hydra effect, whereas pathogen mono-evolution strengthens the hydra effect in both range and magnitude. As the recovery rate increases, we find a counter-intuitive effect of increased disease prevalence due to host-pathogen co-evolution: the disease is first highly infectious and lethal, then highly infectious but with low lethality. Such diverse outcomes suggest that this combined co-evolutionary and epidemiological framework holds great promise for a better understanding of infection.