“…Most of the previous works are concerned with the numerical estimation of the probability of occurrence of epidemics, but the authors know less symbolic solutions such as are given in [5]. For some complex deterministic models with the inclusion of two states of infection, treatment modality, the basic reproduction number was obtained using computer algebra [6,7,8,9]. In such works the symbolic expression of the epidemic threshold was derived, identifying the relationship between the parameters for the occurrence of an epidemic, that is, when R 0 > 1 or when it is controlled, with R 0 <1.…”