Around the time of World War I, a puzzling epidemic of neurological illness began to emerge. Between 1915 and 1928, outbreaks of this condition were documented across the world. Known medically as encephalitis lethargica, it’s also been called “sleepy sickness” (not to be confused with sleeping sickness, spread by the tsetse fly). Its symptoms included fever, seeing double, headaches, and a growing sense of drowsiness. In severe cases, people would fall into comas that they never woke up from, and survivors would often be left with lingering neurological problems.
It’s been estimated that more than a million people may have had encephalitis lethargica and that as many as 500,000 people died from it. Theories for its cause have included toxins, the direct effects of a viral infection (possibly flu) on the brain, and an auto-immune response that shows up after infection, which can be a rare complication for many known infectious diseases.
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