Postencephalitic parkinsonism is a disease that is believed to have been caused by a viral illness, stimulating degeneration of the nerve cells in the substantia nigra, leading to clinical parkinsonism. The disease followed a condition called encephalitis lethargica (Von Economo's encephalitis). There was an epidemic of this in 1918 related to an influenza pandemic - however even using modern molecular diagnostic tests on human material from people who died at the time, no firm link between encephalitis lethargica with influenza has been made.
The film Awakenings (starring Robin Williams and Robert de Niro and based on genuine cases reported by Oliver Sacks) deals sensitively and largely accurately with this disease. The state of the art in treatment remains roughly the same as it was at the time of the events depicted, the 1960s, although patients with postencephalitic parkinsonism lose benefit from their medication far faster than do patients with Parkinson's disease.