Models of disease spread can be very informative in understanding epidemics. In some models one considers for example a situation in which susceptible individuals contract a disease, then recover and become immune to further effects. This model is called SIR for Susceptible - Infected - Recovered. The SIR model follows the populations in three groups of people in time as the disease spreads. The following links provide different levels of the SIR model, and others, from straight forward graphical to deeper mathematical.