Arthritis, or "joint inflammation," refers to a group of diseases that cause pain, swelling, stiffness, and loss of motion in the joints. It is also used as a general term to describe diseases that may affect the joints, but also can cause pain and swelling in other supporting structures of the body, such as muscles, tendons, ligaments, and bones.

Most forms of arthritis are diseases of the immune system, the body’s protection against invasion of infections and toxins. In these diseases, the immune system attacks the body’s own healthy cells, mistaking them for cells that do not belong. This causes inflammation of the lining or connective tissue of the joints, or other parts of the body.

Two types of arthritis are rheumatoid arthritis, affecting more than two million Americans, and psoriatic arthritis, which affects approximately one million Americans.