Normally, the measles virus does not cause brain damage. However, an abnormal immune response to measles or, possibly, certain mutant forms of the virus may cause severe illness and death. This response leads to brain inflammation (swelling and irritation) that may last for years.

SSPE has been reported in all parts of the world, but in western countries it is a rare disease.

Very few cases are seen in the United States since the nationwide measles vaccination program began. SSPE tends to occur several years after a person has measles, even though the person seems to have fully recovered from the illness. Males are more often affected than females. The disease generally occurs in children and adolescents.