  Starting from common assumptions, we build a rate equation model for
multi-strain disease dynamics in terms of immune repertoire classes. We then
move to a strain-level description where a low-order closure reminiscent of a
pair approximation can be applied. We characterize the endemic equilibrium of
the ensuing model in the absence of mutation and discuss the presence of
degeneracy regarding the prevalence of the different strains. Finally we study
the behavior of the system under the injection of mutant strains.
