Idiopathic
Of unknown cause.
Any disease that is of uncertain or unknown origin may be termed idiopathic.
For example, acute idiopathic polyneuritis, diffuse idiopathic skeletal hyperostosis, idiopathic pulmonary fibrosis, idiopathic scoliosis, etc.
From the New Latin idiopathia (primary disease), from the Greek idiopatheia, from idio-, from idios (one's own, personal) + -patheia, -pathic (feeling, suffering).