The third symptom would be recovery, followed by a Parkinson-like syndrome. If patients of Stern followed this course of disease, he diagnosed them with encephalitis lethargica. Stern suspected encephalitis lethargica to be close to poliomyelitis, without evidence. Nevertheless, he experimented with the convalescent serum of survivors of the first acute syndrome. He vaccinated patients with early-stage symptoms, telling them that it might be successful. Stern is author of the definitive 1920s book, Die Epidemische Encephalitis.In 2010, in a substantial Oxford University Press compendium reviewing the historic and contemporary views on EL, its editor, Joel Vilensky, of the Indiana University School of Medicine, quotes another researcher, writing in 1930, who states, "we must confess that etiology is still obscure, the causative agent still unknown, the pathological riddle still unsolved…", and goes on to offer the following conclusion, as of that publication date:

Does the present volume solve the "riddle" of EL, which… has been referred to as the greatest medical mystery of the 20th century? Unfortunately, no: but inroads are certainly made here pertaining to diagnosis, pathology, and even treatment." Subsequent to publication of this compendium, an enterovirus was discovered in encephalitis lethargica cases from the epidemic. In 2012, Oliver Sacks, the author of the book Awakenings, about institutionalized survivors, acknowledged this virus as the probable cause of the disease. Other sources have suggested Streptococcus pneumoniae as a cause. Diagnosis
There have been several proposed diagnostic criteria for encephalitis lethargica.
There is speculation that Adolf Hitler may have had encephalitis lethargica when he was a young adult (in addition to the more substantial case for Parkinsonism in his later years). Mervyn Peake (1911–1968), author of the Gormenghast books, began his decline towards death which was initially attributed to encephalitis lethargica with Parkinsons disease–like symptoms, although others have later suggested his decline in health and eventual death may have been due to Lewy body dementia. Those described in the book Awakenings by the British neurologist Oliver Sacks. Jane Norton Grew Morgan, wife of J. P. Morgan Jr., died of encephalitis lethargica in 1925. At the time, doctors attributed her encephalitis to having contracted influenza during the 1918 pandemic. References
Further reading
Crosby, Molly Caldwell (2010). Asleep: The Forgotten Epidemic that Remains One of Medicines Greatest Mysteries. New York: Penguin/Berkley. – Describes the history of the disease, and the epidemic of the 1920s. Reid, A.H.; McCall, S.; Henry, J.M. ; Taubenberger, J.K. (2001). "Experimenting on the Past: The Enigma of von Economos Encephalitis Lethargica". J. Neuropathol. Exp. Neurol. 60 (7): 663–670. doi:10.1093/jnen/60.7.663. PMID 11444794. Sacks, Oliver (1990). Awakenings. Vintage Books. ISBN 978-0-375-70405-5. OCLC 21910570. Sacks O (1983). "The origin of Awakenings". Br. Med. J. (Clin. Res. Ed.). 287 (6409): 1968–1969. doi:10.1136/bmj.287.6409.1968. PMC 1550182. PMID 6418286. Vilensky, J.A. ; Foley, P.; Gilman, S. (2007). "Children and Encephalitis Lethargica: A Historical Review". Pediatr. Neurol. 37 (2, August): 79–84. doi:10.1016/j.pediatrneurol.2007.04.012. PMID 17675021. Vilensky, Joel A., ed. (2010). Encephalitis Lethargica: During and After the Epidemic. Oxford, UK: Oxford University Press. ISBN 978-0190452209.