  We prove that the seasonally-forced SIR model with a T-periodic forcing has a
periodic solution with period T whenever the basic reproductive number R0>1.
The proof uses the Leray-Schauder degree theory. We also describe some
numerical results in which we compute the T-periodic solution, where in order
to obtain the T-periodic solution when the behavior of the system is
subharmonic or chaotic, we use a Galerkin scheme.
