Appendicitis. Illustration of a human appendix, showing it inflamed with appendicitis. At lower centre, the appendix appears as the narrow finger- shaped tube that branches off the large intestine (upper left). Inflammation of the appendix in appendicitis is a common cause of abdominal pain in children and young adults. The cause is usually not known but it may occur because a lump of faeces is obstructing the appendix, or because of a worm infestation. Symptoms include pain in the lower right-hand side of the abdomen, fever, loss of appetite, and vomiting. Treatment of acute appendicitis is usually appendicectomy, the surgical removal of the appendix.