The SIR model is a simple spread of disease model that shows the spread of influenza in an area where there are no births or deaths, or any immigration or emigration. The name SIR refers to the three groups in the model:susceptible, infected and recovered.
Susceptibles have no immunity to the disease and can catch it. Infecteds have the disease and can spread it to others. Recovereds no longer have the disease and are immune to further infection.
In this model, the user controls the initial number of susceptibles, the rate of infection and the rate of recovery. The flow "get sick," which contains the infection rate, moves susceptibles to the infected category by multiplying susceptibles * infection rate * infecteds. The recover flow moves infecteds to the recovereds category by multiplying the number of infecteds * recovery rate. The user also controls the infection rate and the recovery rate using sliders.
The graph shows the cycle of the illness as people move from being susceptible to being infected to recovering.