On the other hand, infection will persist if where a stable endemic equilibrium exists. The presence of forward bifurcation in some
TB transmission models have been noted (Kribs-Zaleta and Velasco-Hermandez,2000). This is a situation
where the disease-free equilibrium loses its stability and a stable endemic equilibrium appears as .
For such models the requirement is necessary and sufficient for disease elimination. The
phenomenon where a stable endemic equilibrium co-exists with a stable disease-free equilibrium for
is called backward bifurcation and this pattern can be observed in some disease models (Kar and
Modal, 2012). In a backward bifurcation, disease control is only feasible if is reduced further to values
below another sub-threshold less than unity