Have you heard about the strange, horrifying, and unstoppable malady that gripped the planet? Hundreds of thousands lost their lives, and they were the lucky ones. Those who survived remained as hollow shells of their former selves. Zombie-like, they lost all the feelings of life and were condemned to live out their days trapped in their own bodies, speechless, without self-will, barely awake, and with little will to live.

No, we’re not talking about those who listened to the entire performance of William Shatner singing “Mr. Tambourine Man.” We speak, instead of a chapter from not-so-distant history: the Encephalitis lethargica pandemic of 1915-1927. Amazingly, it is a pandemic that is now largely forgotten. One hundred years later, as the world emerges from the grips of another deadly disease, Encephalitis lethargica remains as mysterious as when it first appeared. What’s worse, it could come back at any time.

Austrian neurologist Constantin von Economo was the first to report the malady. He observed patients with extreme lethargy or sleepiness. Those with the acute form of the disease slept all day. Others could remain awake only if they refrained from sitting down. Once seated, they immediately drifted off into slumber. Other symptoms included delirium, headache, paralysis, and abnormal eye movements. Many who recovered from the early stages of the disease entered into a new horror of spending months or years with severe Parkinson-like symptoms that effectively trapped them within their own bodies.

Von Economo called the condition Encephalitis lethargica. Today, it is primarily known as von Economo’s Disease. By the time he identified the illness, the world was in the third year of what would become a global pandemic.

The first victims appeared throughout Europe in the winter of 1915-1916. Often misdiagnosed, and with the lack of coordinated medical communication, the disease made its way across national borders without fanfare. By 1926, however, more than 1 million people contracted Encephalitis lethargica, leaving more than 500,000 dead in its wake.

The height of the pandemic hit as the world was reeling from World War I and its aftermath. The Great War took the lives of about 40 million people. On top of that, it coincided with the Spanish Flu pandemic of 1918-1920, which infected 500 million people and killed between 20 million and 50 million people. This is a big part of the reason why Encephalitis lethargica has largely slipped into the shadows of history.

It remains, however, as one of the greatest medical mysteries of the 20th century. As mysteriously as the disease made its appearance, it disappeared in 1927, leaving scientists no more enlightened about its cause or means of prevention.

Approximately 40% of those who caught the disease died. While many survivors of the pandemic made complete recoveries, the majority were left with significant neurological or psychiatric disorders. Many of these conditions manifested decades after what appeared to be a complete recovery. These symptoms included Parkinsonism, upper body weakness, muscle pains, neck rigidity, vocal tics, and behavioral changes including psychosis.

Some have speculated that Adolf Hitler may have contracted the disease as a young adult, contributing to his Parkinsonism and deplorable mental condition. If true, von Economo’s Disease could be partially responsible for a lot more deaths than initially recorded.

One of the more horrifying side effects of the disease left many survivors in a zombie-like trance. Unable to move or communicate, they lived out the remainder of their lives in a vegetative state. Since this condition carries many similarities to Parkinson’s Disease, physicians have attempted to use treatments from the latter on the former. This produced hopeful and then cruel results.

In the 1960s, survivors who had been in a catatonic state for decades were given the medication Levodopa. This allowed many survivors to experience their first movement and ability to communicate in decades. In a cruel twist of fate, however, patients developed a tolerance to the medicine within a few weeks and returned to their vegetative state, unable to be revived again. This turn of fate inspired the book and movie Awakenings by Oliver Sacks. (Read the article “The Origin of ‘Awakenings’” for Sacks’ account of what inspired him to write the book).

While the true cause of the illness remains unproven, virologist John Oxford made a great discovery in 2004. In looking at then-living survivors of the disease, he determined that 55% first complained of sore throats. In each of those cases, the sore throat was caused by a rare form of streptococcus. In researching the historical record, Oxford found that many of the survivors of the outbreak a century ago first complained about a sore throat, as well. Oxford believes the response to this rare form of strep throat affected the body’s immune system, causing it to attack the brain.

Discovering the cause of the disease may be enlightening, but there still is no effective treatment, cure, or way to prevent it from happening again. Then again, what are the odds that we could have a repeat of conditions where everyone’s immune system is already weakened because of a deadly pandemic, leaving everyone vulnerable to a viral sneak attack?