From the BBC:
Encephalitis Lethargica was a devastating illness that swept the world in the 1920’s.
It attacked the brain, leaving victims like living statues, speechless and motionless.
During the outbreak, nearly a million died, and millions more were left frozen inside their useless bodies, in institutions.
Noone knew what had caused it, or how to treat it.
Austrian neurologist Constantin von Economo noted:
To look at these patients, one would suppose them to be in a state of profound secondary dementia. Emotions are scarcely noticeable in the face but they are mentally intact.
You may have heard of the disease through the movie Awakenings which was based on a book by Oliver Sacks.
Encephalitis Lethargica also has distinction of being one of the few neurological diseases to have songs inspired by it. Fittingly, the three songs (count ’em: 1, 2, 3) I found online were soporific.
(Link obtained from Neil Gaiman’s site.)