The Sleeping Pandemic That Shadowed the Spanish Flu

While the world was watching the Spanish Flu kill 50 million people, a second pandemic was quietly putting people to sleep. Literally. And a century later, medicine still cannot tell you why.

Encephalitis lethargica (EL), nicknamed "sleeping sickness," swept the globe between 1917 and 1930, killing around 500,000 people and leaving hundreds of thousands of survivors permanently and profoundly changed. Austrian neurologist Constantin von Economo first described it in 1917, but the disease resisted tidy classification from the start. Some patients spent all day asleep and could be roused but returned to sleep almost immediately. Others presented with hyperactivity and mania. The same disease. Opposite symptoms.

Survivors often developed chronic complications, including post-encephalitic Parkinsonism, a Parkinson's-like condition causing severe immobility, or psychiatric syndromes leading to profound mental illness. Around 50 percent of those who survived were permanently changed.

The obvious suspect was the Spanish Flu, given the timing. But EL continued to circulate for nearly a decade after the Spanish Flu disappeared, and decades of research on historic brain tissue samples failed to establish any definitive biological link. In 2012, a group of researchers proposed a different culprit: an enterovirus, a type of RNA virus. An autoimmune explanation has also been proposed, though it struggles to account for how widely the disease spread.

The last known survivor, Philip Leather, spent most of his life immobile in a mental hospital and died in 2002. Today the disease is vanishingly rare, with only 80 reported cases in the past 85 years. What caused it remains genuinely unknown.

Medicine filed this one under "we'll figure it out eventually" and then ran out of time.

Read the full story at Popular Mechanics, June 5, 2026

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