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Arthritis literally means "inflammation of a joint"—the place where two bones connect. Most types of arthritis cause pain and stiffness in and around the affected joint or joints. Some types of arthritis, such as rheumatoid arthritis, also affect the immune system and some internal organs of the body. There are more than 100 different types of arthritis, but osteoarthritis, is the most common, affecting nearly 27 million Americans, almost 60% of whom are women. In osteoarthritis, cartilage—the slippery tissue that protects the ends of your bones in the joint—gradually wears down. This can make your knees feel achier than usual after you work out, or leave you feeling stiff and tired when you climb out of bed.