The role of coinfection in multidisease dynamics

We investigate an epidemic model of two diseases. The primary disease is assumed to be a slowly progressing disease, and the density of individuals infected with it is structured by age since infection. Hosts that are already infected with the primary disease can become coinfected with a secondary disease. We show that in addition to the disease-free equilibrium, there exists a unique dominance equilibrium corresponding to each disease. Without coinfection there are no coexistence equilibria; however, with coinfection the number of coexistence equilibria may vary. For some parameter values, there exist two coexistence equilibria. We also observe competitor-mediated oscillatory coexistence. Furthermore, weakly subthreshold ( which occur when exactly one of the reproduction numbers is below one) and strongly subthreshold ( which occur when both reproduction numbers are below one) coexistence equilibria may exist. Some of those are a result of a two-parameter backward bifurcation. Bistability occurs in several regions of the parameter space. Despite the presence of coinfection, coexistence of the two diseases appears possible only for relatively small values of the reproduction numbers-for large values of the reproduction numbers the typical outcome of competition is the dominance of one of the diseases, including bistable dominance where the competition outcome is initial condition dependent.