"The sleepy sickness, as it was called, continued to spread"

Encephalitis lethargica, also known as "von Economo disease" or "sleepy/sleeping sickness," is an atypical form of encephalitis which attacks the brain and leaves the victim speechless and motionless. An epidemic spread around the world between 1915 and 1926, and isolated cases still occur. Not to be confused with the "sleeping sickness" trypanosomiasis, transmitted by the tsetse fly.

Encephalitis lethargica at the National Institue of Neurological Disorders and Stroke.