Now we assume
(1) that new susceptible are introduced at a constant birth rate µ, (2) that the infectious and
recovered classes experience the same constant birth rate µ, and (3) that all the three classes
experience the same constant death rate, equal to the birth rate µ. The third assumption
ensures that the population size is fixed. For simplicity, set N = 1, which can be achieved by
non-dimensionalization. The new SIR model is: