Subacute sclerosing panencephalitis is a very rare complication of measles, occurring about once in every 100,000 cases of measles. In most patients, measles contracted at any age results in immunity to the disease indefinitely. However, in some patients who contract the disease before the age of two whose mothers had no immunity to the disease, the disease goes through an initial acute phase, then develops into a latent stage. Certain of the measles viruses then undergo a mutation which renders them impervious to the body's immune system. After about 6-15 years, the latent virus starts attacking the body's nervous system, particularly the spinal cord and the brain, resulting in severe neurological symptoms.
The disease must be treated soon after symptoms begin to appear, otherwise the course of the disease is terminal (similar to rabies).