<unk>  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<unk> ."
With some medical conditions, the medical community cannot establish a root cause for a large percentage of all cases (e.g. Focal segmental glomerulosclerosis, the majority of which are<unk> ; with other conditions, however,<unk>  cases account for a small percentage (e.g., pulmonary fibrosis). As medical and scientific advances are made with relation to a particular condition or disease, more root causes are discovered, and the percentage of cases designated as<unk>  shrinks.
In his book The Human Body, Isaac Asimov noted a comment about the term<unk> " made in the 20th edition of Stedman's Medical Dictionary: "A high-flown term to conceal ignorance."
Idiopathic
Auto-inflammatory disorders
Radiation enteropathy
Inflammatory bowel disease
Non-Hodgkin lymphoma
Systemic autoimmune diseases
Splenic disease
Glomerular disease