The appendix is usually 8 to 10 cm (3 to 4 inches) long and less than 1.3 cm (0.5 inch) wide. The cavity of the appendix is much narrower where it joins the cecum than it is at its closed end. The appendix has muscular walls that are ordinarily capable of expelling into the cecum the mucous secretions of the appendiceal walls or any of the intestinal contents that have worked their way into the structure. If anything blocks the opening of the appendix or prevents it from expelling its contents into the cecum, appendicitis may occur. The most common obstruction in the opening is a fecalith, a hardened piece of fecal matter. Swelling of the lining of the appendiceal walls themselves can also block the opening. When the appendix is prevented from emptying itself, a series of events occurs. Fluids and its own mucous secretions collect in the appendix, leading to edema, swelling, and the distention of the organ. As the distention increases, the blood vessels of the appendix become closed off, which causes the necrosis (death) of appendiceal tissue. Meanwhile, the bacteria normally found in this part of the intestine begin to propagate in the closed-off pocket, worsening the inflammation. The appendix, weakened by necrosis and subject to increasing pressure from within by the distention, may burst, spilling its contents into the abdominal cavity and infecting the membranes that line the cavity and cover the abdominal organs (see peritonitis). Fortunately, peritonitis is usually prevented by the protective mechanisms of the body. The omentum, a sheet of fatty tissue, often wraps itself around the inflamed appendix, and an exudate that normally develops in the areas of inflammation behaves like glue and seals off the appendix from the surrounding peritoneal cavity.

A person experiencing an attack of appendicitis may feel pain all over the abdomen, only in the upper abdomen, or about the navel. This pain is usually not very severe. After one to six hours or more the pain may become localized to the right lower abdomen. Nausea and vomiting may develop sometime after the onset of the pain. Fever is usually present but is seldom high in the early phases of the attack. The patient's leukocytes (leukocyte) (white blood cells) are usually increased from a normal count of 5,000–10,000 in an adult to an abnormal count of 12,000–20,000; this phenomenon can be caused by many other acute inflammatory conditions that occur in the abdomen.

In a person with a normally sited appendix, the pain of appendicitis is situated at a point between the navel and the front edge of the right hipbone. But many people have the appendix lying in an abnormal position and may feel the pain of an appendicitis attack in a different or misleading location, which makes their symptoms difficult to distinguish from the abdominal pain caused by a variety of other diseases. Careful diagnostic (diagnosis) examination by a physician can usually determine if acute appendicitis is indeed causing a patient's abdominal pain. Ultrasound or computed tomography (CT) scanning may also be useful in the diagnosis of appendicitis.

The basic treatment (therapeutics) of appendicitis is the surgical removal of the appendix in a minor operation called an appendectomy. The operation itself requires little more than a half hour under anesthesia and produces relatively little postoperative discomfort. If a diagnosis of acute appendicitis cannot immediately be made with reasonable certainty, it is common to wait and observe the patient's symptoms for a period from 10 to 24 hours so that a definitive diagnosis can be made. This wait does slightly increase the risk that the appendix will rupture and peritonitis set in, so the patient is kept under careful medical surveillance at this time.