Appendicitis is a painful swelling of the appendix, which is a small, thin pouch attached to the large intestine where feces forms. The exact cause of appendicitis is not fully understood, but it is believed to be caused by a blockage in the lining of the appendix, which can cause an infection. The blockage can be caused by hardened stool, growths, or tumors that block the opening inside the appendix, or by enlarged tissue in the wall of the appendix caused by infection in the digestive tract or somewhere else in the body. In some cases, an upper respiratory tract infection could cause the lymph node within the wall of the bowel to become swollen, leading to inflammation and swelling, which could cause increased pressure within the appendix and eventually lead to a burst appendix. Although anyone can develop appendicitis, it most often occurs in people between the ages of 10 and 30.