Rheumatoid arthritis (RA) is an autoimmune disease that can affect joints, the heart, the skin, and other body organs. Autoimmune diseases such as RA and systemic lupus erythematosus are caused by factors that prompt the body to attack itself. These factors, which normally help fight off infection, target the synovial membrane (joint lining) in RA. These linings are elastic and help the joint maintain its shape; damage to the linings allows the bones to change their position, leading the joint to become deformed. Inflammation can also affect other body parts. In some persons with RA, the autoimmune process affects the heart and its lining (the pericardium) or the lining of the lungs (the pleura). If fluid builds up within the pericardium or pleura, this can be life-threatening and require emergency drainage. Women are more likely to have RA than men. About 1% of the world's population may have RA. The cause of RA is unclear, but it probably has a genetic (inherited) influence in addition to outside (environmental) factors.