Fig. 1

Bifurcation diagram showing fixed points of prevalence of the simple SIS model. The solid blue lines correspond to locally asymptotically stable equilibria. The dashed red lines correspond to unstable equilibria. The disease-free equilibrium point is locally asymptotically stable for \(R_{0} \le 1\) and unstable for \(R_{0} > 1\) (where a transcritical bifurcation occurs at \(R_{0} = 1\)). The endemic equilibrium point is locally asymptotically stable for \(R_{0} > 1\). Here the infectious period is assumed to be 200 days and the transmission parameter, \(\beta\), is varied to provide the appropriate value of \(R_{0}\)