Encephalitis lethargica, between 1915 and 1926; cause unknown

[Astronuc]

I just read about a pandemic of "sleeping sickness" aka Encephalitis lethargica from 1915 to 1926; cause unknown!

https://en.wikipedia.org/wiki/Encephalitis_lethargica

Encephalitis lethargica (EL) is an atypical form of encephalitis. Also known as "von Economo Encephalitis", "sleeping sickness" or "sleepy sickness" (distinct from tsetse fly–transmitted sleeping sickness), it was first described in 1917 by neurologist Constantin von Economo and pathologist Jean-René Cruchet. The disease attacks the brain, leaving some victims in a statue-like condition, speechless and motionless. The exact number of people infected is unknown, but it is estimated that more than one million people contracted the disease during the epidemic, which directly caused more than 500,000 deaths

Encephalitis lethargica is characterized by high fever, sore throat, headache, lethargy, double vision, delayed physical and mental response, sleep inversion and catatonia.

Postencephalitic Parkinsonism was clearly documented to have followed an outbreak of encephalitis lethargica following the 1918 influenza pandemic; evidence for viral causation of the Parkinson's symptoms is circumstantial (epidemiologic, and finding influenza antigens in encephalitis lethargica patients), while evidence arguing against this cause is of the negative sort (for example, lack of viral RNA in postencephalitic Parkinsonian brain material).