Encephalitis lethargica (also known as 'Sleepy Sickness') is a degenerative brain disease first described in 1917.

(NOTE: It should not be confused with 'Sleeping Sickness', which is caused by a parasite.)

The disease attacks the brain, leaving some victims in a statue-like condition, speechless and motionless. Conscious and aware – yet not fully awake.

There was a major global outbreak between 1915 and 1926 affecting around five million people, a third of whom died in the acute stages.

Isolated cases still occur.

The causes of encephalitis lethargica are uncertain. Some studies have explored its origins in an autoimmune response, and, separately or in relation to an immune response, links to pathologies of infectious disease — viral and bacterial.

Source :Wikipedia