An inflammation of the vermiform appendix. Acute appendicitis is the most common cause of emergency abdominal surgery, occurring in 5–6% of the population of the United States. Although about 80% of the individuals are between 5 and 35 years of age, the disease does occur in the very young as well as in the elderly. In the latter groups, the clinical picture is atypical, and frequently results in a significant delay in diagnosis with a higher incidence of complications. Acute appendicitis is rare in children less than 1 year of age.