2.6. Existence of the Endemic Equilibrium. R 0 can also be calculated from the endemic equilibrium, in the case where there is a bifurcation at R 0 = 1 such that the endemic equilibrium does not exist for R 0 < 1. The existence of the endemic equilibrium is, thus, a threshold that can be rearranged to produce an R 0 -like surrogate. However, for many models, calculating the endemic equilibrium can be quite difficult. Furthermore, in the case of a backward bifurcation, the endemic equilibrium still exists for R 0 < 1 (in fact, two endemic equilibria exist in this case, one of which is stable and the other unstable). It follows that the endemic equilibrium is not a useful general method for calculating R 0 and does not always produce the average number of secondary infections.