1919–1930 encephalitis lethargica epidemic

The encephalitis lethargica epidemic lasted from around 1918 to 1930. The cause is still unknown. Though the cause was once attributed to the coinciding Spanish flu epidemic, modern research has disputed this claim. The mortality was as high as 20%

Background
Encephalitis lethargica is a neurological syndrome that causes lethargy, a “mask like” face, excessive blood in the meninges, and other general neurological symptoms. Officially recognized as its own condition in 1917, it is believed to have existed far longer in human history. It is known to cause post-encephalitic parkinsonism.

Timeline
Neurologist Constantin von Economo published a paper in April 1917 on some cases he encountered in the winter months of 1916 and 1917. These patients, despite their various previous diagnoses, had a similar pattern of symptoms. This led him to suggest a novel disease, which he named Encephalitis lethargica. In France, physician René Cruchet was encountering similar patterns, and published his findings within a few days of Constantin von Economo. Following these two reports, many more cases were reported, first in Europe, but quickly spreading around the globe. The epidemic peaked between 1920 and 1929, with an estimated million people diagnosed with Encephalitis lethargica during the epidemic period.

Causes
The causes of encephalitis lethargica are still unknown. Though the connection to the Spanish flu epidemic is often made, the encephalitis outbreak did begin slightly earlier. However, this cannot disprove the claim, simply point out that not all of the existing evidence lines up.

Aftermath
In the aftermath of the epidemic, many cases of post-encephalitic parkinsonism were reported. This condition was distinctive from idiopathic Parkinson's disease, as it occurs in younger patients than typical idiopathic Parkinson's disease, and lacks the "pill-rolling tremor" of idiopathic Parkinson's disease.