"Subacute sclerosing panencephalitis is caused by direct invasion of the brain by mutant measles viruses. There are fewer than 10 cases a year in the U.S. The disease tends to occur years after the patient appears to have recovered from measles.
Symptoms include gradual behavioral change leading to bizarre behavior, muscle jerks, unsteady walking, seizures and frequently death within one to two years."
There is more good stuff in the article and it is not for the faint-hearted.
Hat tip - Peggy Sunny Socal who pens Biology in Science Fiction.