SIR dynamics for the four different contact–density functions. For each simulation run in which there was successful persistence, the 8th year was retained (days 3030 to 3395). The figure shows all these outputs plotted on top of each other. The increase in Susceptibles corresponds with the birth pulse. Infectious period 1/γ = 60 days, transmission rate p = 50, initial population size N0 = 100 000.