Arthritis: Arthritis is inflammation of the joints. Causes include injury, abnormal metabolism (such as gout), inheritance (such as in osteoarthritis), infections (such as in the arthritis of Lyme disease), and an overactive immune system (such as rheumatoid arthritis [RA] and systemic lupus erythematosus). Symptoms of arthritis include pain and limited joint function. Treatment may include pain medications, pain-relieving creams, and dietary supplements.