In the early 20th century, thousands found themselves frozen like ‘statues.’ Experts still don’t know what caused it

Between the winter of 1916 and the early 1930s, a strange and terrifying illness swept across the globe. Known as encephalitis lethargica, or “sleeping sickness,” it immobilized its victims, freezing them into a lifeless, statue-like condition.

The disease was highly unpredictable. While some patients became profoundly unresponsive, their movements slowed to a near halt, and speech vanished entirely, others suffered from hyperkinesia—extreme restlessness and involuntary movements—and debilitating insomnia. Across the board, patients often experienced fever, severe headaches, and paralysis of the eye muscles

The early, baffling cases

The illness first drew medical attention in the winter of 1916-1917, when neurologist Constantin von Economo in Vienna and pathologist Jean-René Cruchet in France documented clusters of baffling cases.

The epidemic peaked in the early 1920s, spreading across Europe and North America. It is estimated to have affected over a million people worldwide. The disease was deadly in its acute stage, with a high mortality rate often estimated at 30% or more.

Survivors faced a cruel, long-term fate: a significant number developed severe neurological complications, sometimes years later, that closely resembled Parkinson’s disease.

No one knows what caused it

Despite decades of study, the origin of this mysterious epidemic remains one of medicine’s most haunting mysteries. The disease coincided closely with the devastating 1918 influenza pandemic, leading many researchers to speculate it was caused by a viral infection related to the flu. However, modern scientific analysis has not confirmed this link. Other theories suggest it was an autoimmune response, where the body’s immune system mistakenly attacked the brain, or a reaction to a different infection, but no single cause has ever been proven.

What makes encephalitis lethargica even more enigmatic is that, while the large-scale epidemic vanished by the 1930s, isolated cases continue to appear worldwide. Each new case reinforces that the mystery of this “sleeping sickness” continues to elude doctors and researchers, a chilling reminder of how little we sometimes understand about the human brain.