(a) The relationship between evolutionarily stable virulence (α) and the relative virulence in the first infected class (ρ) for three levels of infection hazard (solid line, ψ = 0.1; dashed line, ψ = 0.2; dotted line, ψ = 0.4). Parameters are μ = 0.5, γ = 50, σ = 10 and ϕ = 1. (b) The relationship between evolutionarily stable virulence (α) and the relative transmission in the second class (ϕ) for three levels of ρ (solid line, ρ = 0.001; dashed line, ρ = 1; dotted line, ρ = 2). Here, ψ = 0.2.