Post-encephalitic Parkinsonism is a disease believed to be caused by a viral illness that triggers degeneration of the nerve cells in the substantia nigra. Overall, this degeneration leads to clinical parkinsonism.
Historically, starting in 1917 an epidemic of encephalitis lethargica, also called von Economo’s encephalitis or "sleepy-disease" occurred, possibly related to the 1918 Spanish flu pandemic; however, even with the use of modern molecular diagnostic tests on appropriate corpses no firm link between encephalitis lethargica with influenza has been made. Although parkinsonism was occasionally seen during the acute encephalitic phase of encephalitis lethargica, it was often encountered in the post-encephalitic phase. The onset of post encephalitic parkinsonism can be delayed by several years from the resolution of encephalitis lethargica.