Background: simple SIS model Let’s start by refreshing the basics of the SIS model. Susceptible individuals (\(S\)) become infected and move into the infected class (\(I\)), and then infected individuals who recover move straight back to the susceptible class (so there’s no period of immunity like in the SIR model). For simplicity we will ignore births and death, and also use a proportional model i.e. \(S\) + \(I\) = \(N\) = 1.