It's not entirely clear what causes appendicitis.
The appendix is connected to the large intestine, where faeces are formed. It's located in the lower right-hand side of the abdomen (tummy).
Some cases of appendicitis are thought to be caused by a small piece of faeces getting trapped in your appendix and causing a blockage. Bacteria in the appendix then start to multiply, causing it to fill up with pus and swell.
Other potential sources of a blockage include types of inflammatory bowel disease, such as Crohn's disease and ulcerative colitis.
It's also thought that appendicitis may be caused by a stomach infection that has travelled to the appendix.
If the swollen appendix is not removed through surgery, it will eventually burst and the pus may infect other parts of your body.
Read more information about treating appendicitis.
This is dangerous as the bacteria can cause an infection in the abdomen (a condition called peritonitis) and an abscess.
Read more information about the complications of appendicitis.
