A standard SIR (susceptible, infectious, recovered) model was used to simulate MORV transmission , described by the following set of coupled ordinary differential equations: dSdt=B(t) N−βSIN−μ S,dIdt=βSIN−γ I−μ IanddRdt=γ I−μ R, where B(t) is the time-dependent birth function described earlier, μ is the mortality rate, γ is the 1/infectious period and β = cp is the transmission coefficient, which consists of p (rate at which S becomes I when in contact with an I individual) and contact–density function c = f(N/A) that can acquire different shapes depending on population density (explained below).