Idiopathic Parkinson disease generally occurs as a single case in a family - it is a "sporadic" disease. The cause is unknown: environmental factors have been incriminated, but have never been formally identified as "the" cause of Parkinson disease. There are rare cases of familial Parkinson disease due to mutations. Medications (such as neuroleptics), virus as the one responsible for the epidemic of encephalitis lethargica ("sleepy sickness") in 1915 may cause Parkinson syndromes.