Arthritis, the chronic swelling of joints within the body, affects half of all people age 65 and older. The condition can take over 100 different forms, ranging in severity, symptoms, and possible treatments. While researchers have pinpointed causes for some types of arthritis, most remain unexplained. When afflicted with arthritis, a patient can experience pain and loss of movement in nearly any part of the body. The condition is usually chronic, seeping into the joints over a long period of time. The more serious forms can cause swelling, warmth, redness, and pain.

The three most common kinds of arthritis in older people are osteoarthritis, rheumatoid arthritis, and gout. Juvenile arthritis is a type of arthritis that afflicts children under age 16.

• Osteoarthritis, once called degenerative joint disease, is the most common type of arthritis in older people. Symptoms can range from stiffness and mild pain that comes and goes to severe joint pain and even disability. Osteoarthritis often hurts in joints that bear a lot of weight such as the knee, hip, and spine than in the wrist, elbow, and shoulder joints. It is thought of as a condition in which joint injury in genetically-predisposed individuals leads to its development. Joint injury can be chronic and steady, as in obesity, or more intense and episodic, as in repetitive motion disorders. The stronger the genetic predisposition, the less injury needed to establish osteoarthritis, and vice versa.

• Rheumatoid arthritis (RA) can be one of the more disabling forms of arthritis. Signs of RA often include morning stiffness, swelling in three or more joints, swelling of the same joints on both sides of the body (both hands, for example), and bumps (or nodules) under the skin most commonly found near the elbow. It is also likely that people who get RA have certain inherited traits (genes) that cause a disturbance in the immune system.

• Gout occurs most often in older men. It affects the toes, ankles, elbows, wrists, and hands. An acute attack of gout is very painful and usually begins at night with increasing pain. Swelling may cause the skin to pull tightly around the joint and make the area red or purple and very tender. It is a condition in which genetic metabolic abnormalities predispose to uric acid buildup.

The causes of arthritis vary greatly. Osteoarthritis is often brought on by an initial injury that damages the cartilage within a joint, and is compounded by genetic pre-disposition. Obesity is also a risk factor for osteoarthritis. Rheumatoid arthritis is believed to be caused by an auto-immune reaction; that is, the body attacks its own tissues as though they were foreign matter. Gout develops when uric acid crystals lodge in the joints. Many forms of arthritis are related to a combination of genetic, autoimmune, metabolic, and biomechanical factors. However, arthritis remains one of the larger mysteries of modern science.