Figure 1.

Bistability in the susceptible–primed–infectious system when there are no fecundity costs of infectiousness and priming. (a) Region of bistability, when priming rate (p) is high, is dependent on priming protection (q). When priming protection is high (q = 0.1), there is no bistability, but when priming protection is less (q = 0.5), the pathogen can persist even when all susceptible individuals become primed (p = 1). (b) Bistability plot in terms of R₀. R₀ < 1 when p = 0.9 for the model parameters used. When q > 0.1, the number of infected can be positive, with the system at an endemic equilibrium, even if R₀ < 1. Therefore, higher levels of priming protection result in persistence of the pathogen below R₀ < 1. (Parameters: a = 2, b = 1, h = 0.1, α = 1, β = 2, τ = 0, c = 0, f_Y = 1, and f_P = 1. Solid lines represent a stable population and dashed lines represent an unstable population).