More than 50 million Americans have arthritis, a term that encompasses more than a hundred types of joint diseases and conditions. It can affect adults and children, though women and older people are affected more. It can cause pain, swelling, stiffness and permanent joint changes. Some forms affect other parts of the body as well. The most common type is osteoarthritis, which occurs when cartilage wears away, forcing bone to rub against bone. Other forms are inflammatory, due to a wayward immune system; metabolic, due to high levels of uric acid in the body; and infections, caused by bacteria, viruses or fungi.