A general mathematical model for a disease with an exposed (la-tent) period and relapse is proposed. Such a model is appropriate for tuber-culosis, including bovine tuberculosis in cattle and wildlife, and for herpes.For this model with a general probability of remaining in the exposed class,the basic reproduction number R0is identified and its threshold property isdiscussed. In particular, the disease-free equilibrium is proved to be globallyasymptotically stable if R0< 1. If the probability of remaining in the exposedclass is assumed to be negatively exponentially distributed, then R0= 1 is asharp threshold between disease extinction and endemic disease. A delay dif-ferential equation system is obtained if the probability function is assumed tobe a step-function. For this system, the endemic equilibrium is locally asymp-totically stable if R0> 1, and the disease is shown to be uniformly persistentwith the infective population size either approaching or oscillating about theendemic level. Numerical simulations (for parameters appropriate for bovinetuberculosis in cattle) with R0> 1 indicate that solutions tend to this endemicstate