Existence of periodic solutions of a periodic SEIRS model with general incidence

For a family of periodic SEIRS models with general incidence, we prove the existence of at least one endemic periodic orbit when R_0>1. Additionally, we prove the existence of a unique disease-free periodic orbit, that is globally asymptotically stable when R_0<1. In particular, our main result establishes a sharp threshold between existence and non-existence of endemic periodic orbits for this family of models.