Take note: I am not a doctor or scientist of any kind and have almost no idea what I'm taking about. Most of this information comes from Mary Caldwell Crosby's Asleep: The Forgotten Epidemic That Remains One of Medicine's Greatest Mysteries.
Also called von Economo disease, encephalitis lethargica has a wide array of symptoms including high fever, abnormal facial movements, tremors, muscle weakness or rigidity, delayed responses and abnormal sleep patterns, most famously lethargy. A pandemic outbreak hit during the 1920's following the Spanish Flu, although there is evidence of outbreaks happening decades and centuries before it was identified. All reports since then have been isolated incidents.
There is no known cure or cause for encephalitis lethargica, although the current leading theory is that it is an autoimmune disease set off after the body fights off an illness such as strep throat or Spanish Flu. For an encore, the immune system attacks the midsection of the brain, causing swelling, which leads to the other symptoms, so the theory goes.
Despite being a rare and relatively little-known disease, encephalitis lethargica has had a profound impact on history. Aside from the 20th century epidemic, there is evidence that the initial description of schizophrenia was based on patients actually suffering from encephalitis lethargica, which can have mental symptoms such as hallucinations, delusions, or, in one famous instance, self-mutilation. Some believe it infected people in 17th century New England and led to the Salem Witch Trials.
Treatment is almost entirely based on stabilizing the patient and relieving the symptoms. Oliver Sacks famously treated some still-living patients from the 1920's epidemic with L-DOPA , causing some of them to come out of decades-long comas. This was depicted in his book, Awakenings, which was later turned into a film.