Q. What’s the difference between osteoarthritis and rheumatoid arthritis?

A. The term “arthritis” refers to an inflammation of the joints, but there are many types of arthritis that have distinct causes. Osteoarthritis is the most common, and can be thought of as a “wear and tear” of the joints, says Monica Piecyk, a rheumatologist at New England Baptist Hospital. Cartilage cushioning the joints breaks down, leading the bones to rub together and causing pain, swelling, and stiffness. Symptoms often worsen after physical activity.

Osteoarthritis usually progresses slowly, strikes people 40 and older, and becomes increasingly common with age. Piecyk says it may develop around a deformed joint or a previous injury, or because excessive weight puts pressure on the joints.

In contrast, rheumatoid arthritis is an autoimmune disease in which the immune system attacks cartilage around the joints. “Rheumatoid arthritis can be more of a total body disease rather than just a disease of the joints,” Piecyk says. Over time it can lead to fatigue, damage to internal organs, and the growth of nodules under the skin. It’s more common in women and can strike at any age. Symptoms are often worse after sleeping or sitting. It usually develops more quickly than osteoarthritis and Piecyk says it should be treated early to prevent progression.

Treatment for both types of arthritis may involve physical therapy, anti-inflammatory medications, and surgery in some cases, but rheumatoid arthritis is also treated with antirheumatic drugs and biologic drugs that modulate the immune system.