The SIR model is a type of compartmental model used in epidemiology to describe how infectious diseases spread through a population. It divides the population into three groups, or "compartments": susceptible (S), infectious (I), and recovered (R). Individuals move from one compartment to another over time, and this movement is described using a set of coupled differential equations. For example, the rate at which susceptible individuals become infectious depends on how many susceptible and infectious people are currently in the population.
In the plot, each compartment is shown as a curve over time. The susceptible curve starts high and declines as individuals become infected. At the same time, the infectious curve rises, creating an epidemic peak. Once the susceptible population drops below a critical threshold, and as infectious individuals recover, the infectious curve begins to fall. Meanwhile, the recovered curve steadily increases and eventually plateaus, reflecting that most of the population has passed through infection.
