The cause of Paget's disease is unknown, although it might have to do with genes or a viral infection early in life.
The disease occurs worldwide, but is more common in Europe, Australia, and New Zealand.
In people with Paget's disease, there is an abnormal breakdown of bone tissue, followed by abnormal bone formation. The new bone is bigger, but weaker and filled with new blood vessels.
The disease may only be in one or two areas of the skeleton, or throughout the body. It often involves bones of the arms, collarbones, leg, pelvis, spine, and skull.