Fig. 2

The plot describes the backward bifurcation diagram for model (3). The bold red line indicates the stable endemic equilibrium \(D_1\), the red dashed line on the x-axis shows the stable disease-free equilibrium \(D_0\), the blue dashed line on the x-axis represents the unstable \(D_0\), while the blue dashed line on the vertical y-axis indicates the unstable \(D_1\)