First, it's viruses, not virii. And viruses are not the sum totality of infectious diseases. Second, as I mentioned in another comment, a very easy extension o

First, it's viruses, not virii. And viruses are not the sum totality of infectious diseases. Second, as I mentioned in another comment, a very easy extension of this model allows infectious diseases to reach a state where they exist at equilibrium within the population. I have no idea why the authors didn't use them.