We study a susceptible-infected-susceptible model with distributed delays. By constructing suitable Lyapunov functionals, we demonstrate that the global dynamics of this model is fully determined by the basic reproductive ratio R 0 . To be specific, we prove that if R 0 ≤ 1, then the disease-free equilibrium is globally asymptotically stable. On the other hand, if R 0 > 1, then the endemic equilibrium is globally asymptotically stable. It is remarkable that the model dynamics is independent of the probability of immunity lost.