Kawasaki disease is an uncommon illness that mostly affects children under five years of age. It is caused by inflammation of blood vessels throughout the body, including those of the heart (the coronary vessels). This inflammation is known as vasculitis. There have been many theories about what causes this, but no single theory is generally accepted as being correct. It is thought that an infection of some sort causes Kawasaki disease, although this has not been proven. Kawasaki disease is not contagious to other children.