At the beginning of the 20th century, an epidemic of sleeping sickness (or encephalitis lethargica) swept through Europe. The disease broke out in 1916. In 1917, Constantin von Economo (Romanian aristocrat of Macedonian extraction, doctor of psychiatry in Vienna, and Austria's first military pilot) undertook study of the phenomenon after returning to his clinic in the wake of his brother's death in The War.
In January of that year, von Economo began to delineate the common symptoms preceding the final descent into an often-fatal sleep: high temperature, hallucination, impaired vision, and excessive sleepiness.
Approximately a third of patients reporting precursor symptoms fell asleep for extended periods, and were impossible to wake. The majority of these died while still asleep. Others manifested a complete inability to fall asleep, even with the administration of strong sedatives. All of these patients died of extreme fatigue and related side-effects.
A small number of patients sank into a deep sleep from which they neither woke nor died, until neurophysiologst Oliver Sacks found them - years later - and woke them by administration of levodopa. (His book "The Awakenings" and the film of the same name chronicles the tribulations - and horrifying trials - of Dr. Sacks and his patients in this process.)
The disease spread throughout the world and killed more than five million people over the course of 10 years. In 1927 the epidemic disappeared with the same mysterious abruptness in which it appeared.
This form of encephalitis still provides scientists with a resilient mystery, as it can be caused by a variety of agents (virus, bacteria, or spirochete), and symptoms are often interpreted etiologically as sleep disorders rather than viral infection.
While incidence of sleeping sickness has tapered down to a rarity, it is still found occasionally. There is no known cure.
Source: The Enchanted World of Sleep, by Paretz Lavie