Compartmental models simplify the mathematical modelling of infectious diseases. The population is assigned to compartments with labels - for example, S, I, or R, (Susceptible, Infectious, or Recovered). People may progress between compartments. The order of the labels usually shows the flow patterns between the compartments; for example SEIS means susceptible, exposed, infectious, then susceptible again. epidemiological compartmental model http://identifiers.org/doi/10.1007/978-3-540-78911-6_2 https://en.wikipedia.org/wiki/Compartmental_models_in_epidemiology