Figure 5.

We show the critical transmission coefficient ${\beta }_c^q$ (a) and ${\overline{y}}_q^{*}$ (b) as a function of q. Using the values of the parameters given in Table 1, except γ = 0.008405 years^-1, we have q_c = 0, and ${\overline{y}}_0^{*}$. In this set of values the backward bifurcation exists for all q.