Answer: PEP is a disease that results in destruction of the dopamine producing neurons of the substantia nigra following an infection.
After World War I, an epidemic of the disease encephalitis lethargica spread rapidly across much of the world. Encephalitis lethargica caused patients to have intractable hypersomnia, and would be unable to stay awake for any prolonged periods of time. The disease was first studied by the Austrian neurologist Constantin von Economo in 1917. Although it is still unknown what caused the pandemic of encephalitis lethargica, one theory suggests that the outbreak of Spanish flu led to immune system changes that permanently damaged the brain. About a third of all patients died as a result of the disease.
In the aftermath of encephalitis lethargica, some surviving patients had long-lasting effects. These patients had neurological deficits such as lethargy, a complete lack of motive or appetite, and no willingness to move. Oliver Sacks, one of the doctors who studied these patients, described them as being "insubstantial as ghosts, and as passive as zombies." This condition, because of its resemblance to Parkinson's disease, was called post-encephalitic Parkinsonism.
Post-encephalitic Parkinsonism usually presents with the following symptoms:
Hypokinesia, a generalized decrease in motor ability.
Unusual rigidity, or constant muscle tone that resists movement. Relatedly, dystonia also occurs in some patients.
Through working closely with these patients, Sacks discovered that the drug levodopa was able to miraculously reverse the symptoms of Parkinsonism. He describes his success in his autobiographical account, Awakenings.
Cases of post-encephalitic Parkinsonism were more common following the outbreak of encephalitis lethargica. There have been no reported cases of post-encephalitic Parkinsonism since the 1960s, and serological analyses have not found any correlations between influenza virus titers and Parkinsonism (The cause of Parkinson’s disease).