Juvenile arthritis is the term used to describe arthritis in children younger than 16 years. Juvenile arthritis is twice as common in girls as boys and the most common type is juvenile idiopathic arthritis (JIA). Juvenile idiopathic arthritis is an autoimmune disorder affecting the joints of the knee, hands and feet. It causes pain, swelling, stiffness, and feeling of warmth in the joints. The probable causes of JIA include autoimmune condition, genetic factors, and environmental factors.