The SIR (Susceptible-Infected-Removed) model is
$$\dot S = - \frac{\beta I S}{N},$$ $$\dot I = \frac{\beta I S}{N} - \gamma I,$$ $$\dot R = \gamma I.$$Parameter $\beta$ is the infection rate, $\gamma$ is the recovery rate. The basic reproduction number $R_0 = \beta/\gamma$.