“…To that end, we use the Nonstandard Finite Difference (NSFD) method, which was initiated more than two decades ago by R.E. Mickens and has, since the publication of his monograph [44], shown great potential in replicating the dynamics of systems that arise in a variety of areas in science and engineering (see, e.g., the edited volumes [46,47], the papers [4,7,25,26], and also [31] for an overview and applications in Biosciences). Furthermore, the usefulness and reliability of the NSFD method in epidemiology is attested by the ever growing number of works over the past few years (see, e.g., [21,22,27,28,53]).…”