Encephalitis Lethargica: The Sleepy Sickness Mystery

Exploring the History, Causes, and Future Risks of a Forgotten Epidemic

Encephalitis lethargica (EL) is known as “sleepy sickness”.  It is a rare and mysterious disease that has perplexed the medical community for over a century. Also, it is characterized by high fever, headache, double vision, delayed response, and lethargy. EL attacks the brain, leaving some victims in a statue-like, speechless, and motionless state. This article delves into the history of EL, explores theories behind its causes, and examines the possibility of its resurgence.

The Mysterious History of Encephalitis Lethargica

EL was first officially recognized in 1917, but historical records suggest similar epidemics as far back as 1529. Notable occurrences include the English sweats (1529), mal mazzuco (Italy, 1597). Also several outbreaks in the 17th and 18th centuries, characterized by symptoms resembling those of EL. However, it was the pandemic between 1915 and 1926 that truly highlighted the disease’s devastating impact. Nearly affecting 5 million people and resulting in approximately 1.6 million deaths globally.

Encephalitis Lethargica, the mysterious disease

Theories and Causes of the Epidemic

The exact cause of EL remains a subject of debate. Initially, it was believed to be connected to the Spanish flu epidemic. Modern research argues against this theory however. Instead, some studies suggest autoimmune responses or links to infectious diseases, such as influenza, as potential causes. Interestingly, post-encephalitic Parkinsonism, a condition following EL, has been documented, although its viral causation remains speculative.

Modern Understanding and Treatment

Today, treatments for EL include immunomodulating therapies and symptom-specific treatments. Despite advances in medical science, the disease’s etiology remains elusive. To this day, occasional cases still emerge, keeping the mystery of EL alive.

Could Encephalitis Lethargica Return?

Considering the enigmatic nature of EL, it is challenging to predict its potential return. The lack of a definitive cause and the sporadic nature of past outbreaks make it difficult to ascertain the likelihood of a future epidemic. However, the historical impact and the sporadic cases observed today suggest that the medical community should remain vigilant.

Encephalitis lethargica brain scan


Encephalitis lethargica remains one of the 20th century’s great medical mysteries. Its sporadic nature and the uncertainty surrounding its causes continue to intrigue and challenge scientists and historians alike. While the likelihood of another pandemic seems low, the unpredictable nature of this disease means it cannot be completely ruled out.

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