Ordinarily, the measles virus does not cause brain damage, but an abnormal immune response to measles or, possibly, certain mutant forms of the virus may cause severe illness and death. This 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 considered a rare disease.
Fewer than 10 cases per year are reported in the United States, a dramatic decrease that followed the nationwide measles immunization program. However, in some countries like India, over 20 cases of SSPE per million people are reported each year.
SSPE tends to occur several years after an individual has measles, even though the person seems to have fully recovered from the illness. Males are more often affected than females, and the disease generally occurs in children and adolescents.