2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/60705An SIRS type model of disease transmission in an open environment is discussed. We use the Poincaré index together with a perturbation method to show that the endemic proportions need not be unique.15 pages, 3 figuresClassical Analysis and ODEsQuantitative Biology92D30text