The S model assumes people carry lifelong immunity to a disease upon recovery; thisis the case for a variety of diseases. For another class of airborne diseases, for example seasonalinfluenza, an individual’s immunity may wane over time. In this case, the SIRS model is usedallow recovered individuals return to a susceptible state, is the rate which recovered individualsreturn to the susceptible statue due to loss of immunity. We can draw the scheme of the model: