Idiopathic

adj. having unknown causes (of a disease)

Idiopathic is an adjective used primarily in medicine meaning arising spontaneously or from an obscure or unknown cause. From Greek ἴδιος, idios (one's own) + παθος, pathos (suffering), it means approximately "a disease of its own kind."It is technically a term from nosology, the classification of disease. For most medical conditions, one or more causes are somewhat understood, but in a certain percentage of people with the condition, the cause may not be readily apparent or characterized. In these cases, the origin of the condition is said to be "idiopathic."
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Adjective
1. (of diseases) arising from an unknown cause; "idiopathic epilepsy"
(pertainym) idiopathic disease, idiopathic disorder, idiopathy

(a.)
Alt. of Idiopathical
  

The cause is unknown.