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 some condition related to R0 holds. Additionally, we prove the existence of a unique disease-free periodic orbit, that is globally asymptotically stable when R0<1. In particular, our main result generalizes the one in Zhang et al. (2012). We also discuss some examples where our results apply and show that, in some particular situations, we have a sharp threshold between existence and non existence of an endemic periodic orbit.