Q:  Is arthritis just one disease and is it hereditary?

A:  Some forms of arthritis, like osteoarthritis, appear to run in families. However, there are more than a hundred forms of arthritis and the causes for many of these are not clear.

Arthritis affects one in seven Americans. Odds are high that someone in each generation of a family will feel it. The two most common forms are osteoarthritis (commonly referred to just as arthritis) and rheumatoid arthritis. The less common forms include gout, lupus, and scleroderma.

People often confuse the two common varieties. Osteoarthritis is an unfortunate infirmity that usually surfaces with age, like hardening of the arteries. It can show up in young people (prompted by heredity, abnormal metabolism, or injury), but most of its victims tend to be older.

Everyone can get osteoarthritis, but females are affected more often than males. Despite the popular image, your joints are not like wheel bearings on a car, which wear out after a certain number of miles.

Osteoarthritis is not the inevitable result of years of activity, and regular exercisers are not necessarily more prone to getting it than couch potatoes. In fact, they may be less prone. Inactivity may be as much a culprit as overusing a joint.

Scientists have found that arthritic joints have a high concentration of cartilage-corrosive enzymes. They believe that arthritis may be caused by joint injuries, muscle weakness, and being overweight, among other factors.

Arthritis most often targets knees, fingers, feet, and hips; as they lose the lubricating, soft cushion between their hard bones, they become inflamed and cause pain. Pain also may stem from the stretching of the bone coating, stretched membranes between joints, or from trapped nerves. Yet another source of pain can be the achy, knobby, bony growths that develop in hands and fingers, giving them a contracted immovable joint.

Rheumatoid arthritis is also an insidious disease, being a reaction of the body's immune system against its own tissue. For unknown reasons, the immune system attacks the joints' soft tissues as if they were an invader such as a bacteria or virus. Rheumatoid arthritis is much less common than osteoarthritis. Approximately three million Americans suffer from rheumatoid arthritis, and women are two to three times more likely to have it than men.

Unlike osteoarthritis, it also assails people at any age, although it tends to appear between the ages of twenty and fifty. In most people, it attacks the hands. However, rheumatoid arthritis travels throughout the body and can affect any of the body's more than three hundred joints.

Scott Fishman, M.D., is a leading expert in pain management.