The characteristic disease features of measles--fever and rash--are associated with the immune response to infection and are coincident with virus clearance. MV-specific antibody and CD4 and CD8 T cell responses are generated and contribute to virus clearance and protection from reinfection. During this same phase of immune activation immunologic abnormalities are also apparent. There is a generalized suppression of cellular immune responses that may contribute to increased susceptibility to other infections. Autoimmune disease may appear in the form of acute disseminated encephalomyelitis. If virus-specific immune responses are inadequate infection may progress with pulmonary or CNS manifestations, but without a rash. The pathogenesis of the rare disease SSPE, that occurs many years after primary infection is not clear, but immune responses show increased antibody to measles and cellular immune responses similar to those seen after uncomplicated infection.