Past exam of the mathematics course of the University of Cambridge / 2006 / ii / Paper 3 / 6B / Solution

The infection term $rIS$ is a mass-action contact rate: each infection removes one susceptible individual and adds one infected individual. Recovery at rate $aI$ transfers individuals from the infected class to the recovered class. Loss of immunity at rate $pR$ transfers recovered individuals back to the susceptible class. Thus these equations describe the SIR model with waning immunity, also called the SIRS model. Adding the equations gives $\dot{S}+\dot{I}+\dot{R}=0$, so $S+I+R=N$. The nonnegative population region is invariant because the derivative of any population on its zero boundary is nonnegative.