We study an SEI model of disease transmission with immigration into all three classes. For incidence, we allow for a nonlinear response to the number of infectives, including mass action and saturating incidence as special cases. There is no disease-free equilibrium and therefore no basic reproduction number. For all parameter values, the only equilibrium is an endemic equilibrium. Using a Lyapunov function, we show that this equilibrium is globally asymptotically stable. (C) 2014 Elsevier Inc. All rights reserved.