This paper applies the sliding mode control to a SEIR epidemic model. Initially, the controller is designed assuming certain knowledge on the upper-bounding of the uncertainty signal. Then, this condition is removed while an adaptive sliding control system is designed. The closed-loop system is able to achieve the control objective regardless the parametric uncertainties of the model and the lack of a priori knowledge on the system. Thus, all the population tends to be immune while the susceptible, infective and infectious tend to zero.