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Beginning as early as 1916, and continuing well into the 1920s, an unusual and disturbing illness devastated millions of people throughout the world. It arrived in the shadow of the 1918 Spanish Flu epidemic⁠—which killed an estimated fifty million people worldwide⁠—so it has been largely overlooked by history despite the fact that it took the lives of over a million people, and left countless others frozen inside unresponsive bodies.

Young people, particularly women, were the most vulnerable to the disease, though it affected people of all ages. When an individual was stricken, the first signs were typically a sore throat and fever accompanied by a headache; but these discomforts soon developed into more alarming problems such as double-vision and severe weakness. Within hours, most of the victims were gripped by episodes of tremors, strange bodily movements, intense muscle pains, and delayed mental response. Symptoms rapidly increased in severity, and in spite of medical attention, most patients worsened dramatically. Behavioral changes often appeared⁠—including psychosis and hallucinations⁠—followed by steadily increasing drowsiness and lethargy. Many became comatose and completely unresponsive.

Medical science was baffled by the bizarre epidemic, which affected millions of people across the globe. The mysterious disease was given the name Encephalitis lethargica, which literally means “inflammation of the brain that makes you tired,” but it was more commonly known as “sleepy sickness.” Such a melancholy designation was appropriate, considering that hundreds of thousands of people died from the inexplicable ailment without ever regaining consciousness.

Among the survivors, victims tended to remain in a coma indefinitely, sometimes for months or years. Although full recoveries were not unheard of, they were a rarity. Many of those stricken with the disease experienced ill effects which lingered throughout the rest of their lives, including vision problems, difficulty swallowing, personality changes, and sometimes permanent psychosis.

One very common problem to befall people recovering from the sleepy sickness was Postencephalitic Parkinson’s disease. This caused life-long symptoms such as slowness, tremors, speech problems, and abnormal muscle movement. In some cases, individuals retained their hearing, intelligence, and reasoning, but were left in a catatonic state, unable to respond to stimuli. This parkinsonism sometimes took up to a year to appear in recovered patients.

In 1928, as suddenly as it had appeared, the encephalitis lethargica epidemic was gone. Although new cases stopped being reported, thousands of those affected were housed in institutions for decades, alive but trapped within useless bodies. In 1969, over forty years after the strange disease disappeared, some catatonic victims were treated with a newly developed antiparkinson drug called Levodopa. A number of patients improved dramatically upon treatment⁠—they stood up from their wheelchairs and became conscious, responsive, and aware of the world around them⁠—but it was soon evident that their miraculous recovery was tragically short-lived. Most patients slipped back into a catatonic state within days or weeks, and repeated dosages were useless. The 1990 movie Awakenings is based on such experiences described in the memoirs of Dr. Oliver Sacks.

MRI of a somnolent patient showing inflammatory regions.

Cases of encephalitis lethargica since the original epidemic have been scarce, but in 1993 a twenty-three year old woman was hospitalized after suffering fever, tremors, hallucinations, and strange arm movements. Her brain was dangerously inflamed, and to the doctors’ surprise, the cause was ultimately determined to be the sleepy sickness. The cause of the original 1916-1928 outbreak had never been determined in the intervening years, so a virologist named Professor John Oxford re-examined brains samples taken from victims of the original epidemic. Despite his advanced molecular probes, he found no evidence of viruses in the tissue.

As the young woman slowly recovered, Doctors Russell Dale and Andrew Church set to investigating the disease, and through the medical community they found twenty other patients with symptoms of encephalitis lethargica. After they analyzed the patients, they discovered a common thread which was also present in the historical cases: most patients complained of a sore throat before the disease struck. The men narrowed the common thread down to a particular strain of bacteria called diplococcus, known to cause sore throats. Though the evidence is insufficient to be certain, the findings of these researchers strongly suggests that the sleepy sickness epidemic was caused by the body’s massive over-reaction to these bacteria. It seems that this excessive immune response caused the immune system to attack the nerve cells of the brain, resulting in significant damage. Further research has detected anti-brain antibodies present in those with the condition, further supporting the auto-immune theory.

Given the new evidence, some experts suggest that encephalitis lethargica may be much more common than we realize. It is likely that most cases are minor, and go undiagnosed. Oxford, Dale, and Church may very well have solved one of medicine’s greatest mysteries, though some researchers still suspect that a virus is responsible for the disease. Research continues.

Note: A previous version of this article referred to the condition as “sleeping sickness,” however current naming conventions prefer “sleepy sickness” for Encephalitis lethargica to differentiate it from African trypanosomiasis.