Numerical simulations of the model defined in Table 1. (a) A homogeneous meningococcal population. Parameters γ = α = 10, β = 12.5, Ψ = 1,000, ε = 0.0008, N = 1,000. (b) A heterogeneous bacterial population. Parameters as in a but with μ = 0.0008 and ε = 0.04. (c) A homogeneous bacterial population with a sinusoidal transmission rate with a period of 1 year. Parameters are γ = α = 13, β = 16.25 ± 4.875, Ψ = 15,600, ε = 0.0000192, N = 5,000,000. (d) A heterogeneous bacterial population with sinusoidal transmission rates. Parameters as in c but with μ = 0.0000192 and ε = 0.016.