Appendicitis is caused by the opening of the appendix getting blocked, causing the appendix to become swollen. As it gets swollen, bacteria inside the appendix can enter the damaged walls, causing infection. The appendix can burst (perforate), causing the bacteria to spread outside the gut and into the tummy (abdominal cavity) more widely.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/what-causes-appendicitis