An anal abscess is an infected cavity filled with pus found near the anus (the opening of the anal canal) or rectum (the portion of large intestine just proximal to the anal canal). This can results from a blockage of the anal glands located just inside the anus. According to the crypto-glandular theory, abscesses often develop from cryptitis which may be associated with an enlarged papillae in the anal canal. They start as cellulitis-a diffuse inflammation, characterized by swelling and redness, which has not yet localized to form an abscess. Then the infecting organisms burrow into the anal glands, producing circumscribed areas of pus in the region of the anus and rectum.

An abscess produces pain and swelling near the anal opening. Fever may also be present. Treatment consists of surgically draining pus from the infected cavity and making an opening (incision) in the skin near the anus to relieve pressure. Sometimes a small catheter is left in the incision for several days to assure adequate drainage. In the majority of individuals, a fistula will form after the abscess has been drained.