Includes both diseases of unknown aetiology, with no established therapeutic remedies (and with the possibility of sequelae), and chronic diseases, which put a heavy social and economic burden on both the patient and the relatives. The definition may be narrowed to include only serious mental and physical disorders (progressive muscular atrophy and dystropy, etc), but may be extended to include: Behcet's disease, multiple sclerosis, myasthenia gravis, and others.