Appendicitis occurs when there is infection or inflammation of the appendix, a tubular structure at the lower end of the caecum. This is a pouch situated at the beginning of the colon, just below where the small intestine enters the colon. In most people, the appendix lies in the lower right side of the abdomen.

Appendicitis is believed to be due to blockage of the lumen of the appendix, usually by a lump of hard stools. The mucus produced by the lining of the appendix then becomes trapped and the appendix becomes swollen. The pressure affects the blood flowing in the wall of the appendix, and eventually causes it to break and burst open, releasing all the infected material into the abdominal cavity.

If there is still no treatment, the infection usually accumulates around the area and causes a large abscess. In some cases, the body is unable to contain the abscess and the pus spreads throughout the entire abdominal cavity, causing severe infection and possibly death.