Measles virus usually doesn't cause brain damage, but certain mutant forms can invade the brain, causing severe illness and death. 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, decreasing the frequency of this disease dramatically, following the nationwide measles immunization program. However, in some countries like India, over 20 cases of SSPE per million people are annually reported.

SSPE tends to occur several years after having measles (rubeola), even though recovery from the illness appears to have been normal. More males are affected than females, and the disease generally occurs in children and adolescents.

Affected individuals frequently die one to two years after being diagnosed with this condition, but some may survive for longer periods.

SSPE is due to the direct invasion of brain cells by the mutant measles virus, which provokes brain inflammation (swelling and irritation with presence of extra immune cells) that can last for years.