A SIRS epidemiological model with two subpopulations and vital dynamics is analyzed. Both subpopulations sizes are considered constant by assuming that the birth and the death rates are equal. We consider the case where one subpopulation is a core, that is a very infectious small group, responsible for a big fraction of the incidence. For this case thresholds are determined and the main equilibrium points for the four dimensional system are shown to be globally stable by using a known Theorem of Markus on asymptotically autonomous systems. This system models the dynamics of typhoid fever , where the core is the group of food handlers . The results presented in this work are an extension of those presented in [3].