Figure 1.

Transition from the three possible states, susceptible $S\left(t\right),$ infected, $I\left(t\right)$, and removed $R\left(t\right).$ The proportion of interactions between individuals in $S\left(t\right)$ and $I\left(t\right)$ that lead to infection per unit time is $r$, and the proportion of infected individuals that recover with immunity or die from disease per unit time is $\gamma$. The total number of individuals within the system remains constant throughout calculation.