dR/dt = γI merely means that the number of recovered individuals in the population, at a given moment, reflects the number of infected individuals times the average recovery rate. So much for R, the “recovered” class. The equations for S (“susceptibles”) and I (“infected”) are likewise opaque but sensible. All this became known as an SIR model. It was a handy tool for thinking about infectious outbreaks, still widely used by disease theorists.
A look into the SIR disease model
