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%.

The epidemic is thought to have started in Romania during 1915, and the disease supposedly spread through the movement of the various troops during World War I. In 1917, it reached the epidemic status in Vienna, where it was first described. Disease outbreaks of encephalitis were first reported in England and France during 1918, followed by Canada, Central America, India, and the United States in 1919. Disease outbreaks were also reported in Germany and the Soviet Union during 1920. The epidemic peaked between 1920 and 1929, with an estimated one million people diagnosed with encephalitis lethargica during the epidemic period. The disease suddenly disappeared in the beginning of the 1930s.

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.