Appendicitis, uh pehn duh SY tihs, is an inflammation of the vermiform appendix (see Appendix). It results from an infection caused by bacteria. The appendix becomes swollen and fills with pus. The pus may be walled off and form an abscess. Or the appendix may break, allowing the infection to spread to surrounding body organs. It may also cause peritonitis, an inflammation of the membrane that lines the abdominal cavity (see Peritonitis).