A rare or orphan disease is a disease which affects relatively few people. The exact definition varies between 1:1000 and 1:200.000. E.g. it can be 1:1500 (USA) , 1:2000 (EU) or 1:2500 (Japan). Rare diseases are mostly caused by genetic variation making them chronic and hard to cure. The severity depends on the affected gene and its physiological implications.
Rare disease pathways on WikiPathways: