What caused encephalitis lethargica, the 'sleepy sickness' of 1917 to 1928?

An epidemic brain disease that left victims in a stupor and many survivors with parkinsonism for decades (Sacks, Awakenings). First described in Vienna (von Economo) and France in winter 1916 to 1917, before the 1918 flu pandemic; it faded by about 1930. The 1918 influenza is the leading suspect, but brain samples have not shown flu virus, and reviews call the link unproven. Ravenholt and Foege (1982) found that in Seattle encephalitis peaks trailed flu peaks by about 12 months.

Where it stands (25 October 2026): in national death records of three countries (United States, England and Wales, Scotland), sleepy sickness did not rise in the 1918 pandemic year, does not follow flu waves a year later, and does not rise with later flu epidemics. Together with the first cases in 1916 to 1917, the epidemic curves give no support to the 1918 flu as its cause; they do not rule out an earlier, milder flu strain or an unrelated virus.