Exercises and notes:

* Work out the differential equations for the SIR model, based on the pumping principles and the law of mass action I described in comment 9. You can check your answers with the SIR equations given in the Wikipedia article on [Compartmental models in epidemiology](https://en.wikipedia.org/wiki/Compartmental_models_in_epidemiology).

* Do the same for the simpler SI model, which is obtained from SIR by removing R and the recovery transition. Compare your results to the [[Logistic equation]].

* See [this discussion](https://forum.azimuthproject.org/discussion/377/logistic-equation-with-recent-commentary-on-corona-virus) on the forum, regarding the logistic equation and COVID-19.