Encephalitis lethargica

Encephalitis Lethargica is a medical condition known on Earth and other planets. It is a deadly, unusual form of encephalitis or sleeping sickness.

An epidemic occurred on Earth in the early 20th century.

History and specifics
In 2274, Leonard McCoy thought Zeta-Atez's Imperator Dykranus might have Encephalitis Lethargica. Christine Chapel thought the symptoms also fit Regulan fever. Dykranus seemed to remain awake for only an hour or two at a time. McCoy later thought the imperator's behavior suggested he was schizophrenic. Eventually his sleepiness was found to be a symptom of bicephaly.