Arthritis is a joint disorder featuring inflammation. A joint is an area of the body where two different bones meet. A joint functions to move the body parts connected by its bones. Arthritis literally means inflammation of one or more joints.

Arthritis is frequently accompanied by joint pain. Joint pain is referred to as arthralgia.

There are many types of arthritis (over 100 identified, and the number is growing). The types range from those related to wear and tear of cartilage (such asosteoarthritis) to those associated with inflammation resulting from an overactive immune system (such asrheumatoid arthritis). Together, the many types of arthritis make up the most common chronic illness in the United States.

The causes of arthritis depend on the form of arthritis. Causes include injury (leading to osteoarthritis), metabolic abnormalities (such as gout andpseudogout), hereditary factors, the direct and indirect effect of infections (bacterial and viral), and a misdirected immune system with autoimmunity (such as in rheumatoid arthritis andsystemic lupus erythematosus).

Arthritis is classified as one of the rheumatic diseases. These are conditions that are different individual illnesses, with differing features, treatments, complications, and prognoses. They are similar in that they have a tendency to affect the joints, muscles, ligaments, cartilage, and tendons, and many have the potential to affect other internal body areas.