100 Years Ago, Thousands Fell Into a Deep Sleep With No Explanation. Today, It’s Still a Mystery. - AOL.com

Here’s what you’ll learn when you read this story:Encephalitis lethargica (EL) is a mysterious neurological disease that impacted millions of people from 1917 to 1930, causing a wide range of symptoms including hypersomnia, immobility, and even death.Despite a century of scientists analyzing the disease, experts aren’t sure what exactly caused the illness, nor do they know for certain if it could return.Because the disease coincided with the Spanish Influenza pandemic for a time, theories suggest there might be a connection, but a more modern analysis looked at other causes, including enteroviruses and autoimmune disorders.A little more than a century ago, the world was fighting a pandemic you know—and one you almost certainly don’t. The famous one was the Spanish Influenza (which actually originated from the U.S.), which ravaged the world from 1918 until 1920. In just two short years, the H1N1 subtype of the influenza A virus killed more than 50 million people worldwide and afflicted at least one-fifth of all humans on Earth.However, at precisely the same time, a less deadly (but all-around stranger) disease was quietly working its way around the globe. This was encephalitis lethargica (EL), a neurological syndrome that was also known as “sleeping sickness” because its symptoms often included hypersomnia—patients essentially spent all day sleeping. Although not nearly as deadly as the Spanish Flu, the EL pandemic still killed roughly five hundred thousand people from 1917 and 1930 and seriously impaired hundreds of thousands of others.While the world mostly forgot about the pandemic after it passed, scientists today still consider the little-known disease as one of the greatest medical mysteries of the 20th century. Over a hundred years after the Austrian physician Constantin von Economo first described it in 1917, we still aren’t certain what causes it, how it’s transmitted, or (crucially) if a similar pandemic could one day happen again.One of the confounding aspects of the EL pandemic was that the disease didn’t always progress the same way within different populations. For example, in some cases, EL killed patients quickly. In others, it affected victims over a prolonged period. Sometimes it caused hypersomnia, and other times it instigated increased levels of activity. The amorphous nature of encephalitis lethargica’s symptoms is reflected in a description that von Economo wrote during the earliest stages of the outbreak.“We are dealing with a kind of sleeping sickness, having an unusually prolonged course,” he wrote. “The first symptoms are usually acute, with headaches and malaise. Then a state of somnolence appears, often associated with active delirium from which the patient can be awakened easily. This delirious somnolence can lead to death, rapidly, or over the course of a few weeks. On the other hand it can persist unchanged for weeks or even months with periods lasting hours or days or even longer.”Von Economo eventually published dozens of papers on EL and kickstarted the century-long scientific effort to better understand this perplexing malady. According to the American Society of Microbiology, EL can be characterized as consisting of two phases. In the acute phase, as indicated in von Economo’s first reports about the disease, flu-like symptoms appear and are followed by extreme sleepiness or (in some cases) mania. If the patient survived this phase, then a second chronic phase, often featuring Parkinsonism, set in. Around 50 percent of those who survived EL were left permanently changed from the experience (percentages were even higher among children), including complete personality changes or even psychosis.One prominent theory about what caused this pandemic emphasized its similarity to the Spanish Flu. The primary evidence that the two diseases were related is the sheer coincidence of them both appearing at the same time and in the same region. Suggestively, reports of encephalitis were also associated with an influenza epidemic from 1889 to 1892 whose victims experienced other neurological symptoms similar to those caused by EL.However, EL continued to persist for nearly ten years after the disappearance of the Spanish Flu, and over the decades, studies focused on historic brain samples of EL patients failed to establish a definitive link between the two diseases. Instead, in 2012, group of researchers suggested that the villain might be an enterovirus (a kind of RNA virus). The poliovirus is an example of an enterovirus, and other strains have been known to induce a condition where muscles relax and weaken. Other theories have suggested that an autoimmune disorder could be to blame, though such an explanation is hard to justify given how widely the disease spread.Scientists still aren’t certain of its cause, but the effects of EL were devastating. The last known survivor of the EL pandemic, Philip Leather, passed away at the age 82 in 2002. He spent most of his life immobile in a mental hospital. Today, the disease is vanishingly rare, with the American Society for Microbiology reporting that there have only been 80 reported cases in the past 85 years.This mysterious disease is ill-remembered, but it remains to be seen if it will remain a historical oddity—or return with an unexpected vengeance.