Arthritis (from Greek arthro-, joint + -itis, inflammation; plural: arthritides) is a form of joint disorder that involves inflammation of one or more joints. There are over 100 different forms of arthritis. The most common form, osteoarthritis (degenerative joint disease), is a result of trauma to the joint, or infection of the joint. Other arthritis forms are rheumatoid arthritis, psoriatic arthritis, and related autoimmune diseases.