Paget disease of the bone is a chronic bone disorder. It causes bones to become enlarged and deformed. Bone may become dense, but fragile, because of excessive breakdown and deformation of bone. The disease is the most common bone disorder after osteoporosis in adults older than age 50. The exact cause of Paget disease of the bone is unknown. It may be due to a slow viral infection of bone. Genetics may also play a significant role.