A tooth abscess or root abscess is pus enclosed in the tissues of the jaw bone at the apex of an infected tooth's root(s). Usually the abscess originates from a bacterial infection that has accumulated in the soft, often dead, pulp of the tooth. This can be caused by tooth decay, broken teeth or extensive periodontal disease (or combinations of these factors). A failed root canal treatment may also create a similar abscess.
Examples of types of dental abscess include gingival abscesses, which involve only the gum tissue, without affecting either the tooth or the periodontal ligament, periapical abscesses which start at the apex of the root, and periodontal abscesses which begin in the pocket of gingiva over 3 mm.
Pain and symptoms 
The pain is continuous and may be described as extreme, gnawing, sharp, shooting, or throbbing. Putting pressure or warmth on the tooth may induce extreme pain. There may be a swelling present at either the base of the tooth, the gum, and/or the cheek, which can be reduced by applying ice packs.
An acute abscess may be painless but still have a swelling present on the gum. It is important to get anything that presents like this checked by a dental professional as it may become chronic later.
In some cases, a tooth abscess may perforate bone and start draining into the surrounding tissues creating local facial swelling. In some cases, the lymph glands in the neck will become swollen and tender in response to the infection. It may even feel like a migraine as the pain can transfer from the infected area. The pain does not normally transfer across the face, only upwards or downwards as the nerves that serve each side of the face are separate.
Severe aching and discomfort on the side of the face where the tooth is infected is also fairly common, with the tooth itself becoming unbearable to touch due to extreme amounts of pain.
Successful treatment of a dental abscess centers on the reduction and elimination of the offending organisms. This can include treatment with antibiotics and drainage. If the tooth can be restored, root canal therapy can be performed. Non-restorable teeth must be extracted, followed by curettage of all apical soft tissue.
Unless they are symptomatic, teeth treated with root canal therapy should be evaluated at 1- and 2-years intervals to rule out possible lesional enlargement and to ensure appropriate healing.
Abscesses may fail to heal for several reasons:
- Cyst formation
- Inadequate root canal therapy
- Vertical root fractures
- Foreign material in the lesion
- Associated periodontal disease
- Penetration of the maxillary sinus
Following conventional, adequate root canal therapy, abscesses that do not heal or enlarge are often treated with surgery and filling the root tips; and will require a biopsy to evaluate the diagnosis.
Untreated consequences 
If left untreated, a severe tooth abscess may become large enough to perforate bone and extend into the soft tissue eventually becoming osteomyelitis and cellulitis respectively. From there it follows the path of least resistance and may spread either internally or externally. The path of the infection is influenced by such things as the location of the infected tooth and the thickness of the bone, muscle and fascia attachments.
External drainage may begin as a boil which bursts allowing pus drainage from the abscess, intraorally (usually through the gum) or extra orally. Chronic drainage will allow an epithelial lining to form in this communication to form a pus draining canal (fistula). Sometimes this type of drainage will immediately relieve some of the painful symptoms associated with the pressure.
Internal drainage is of more concern as growing infection makes space within the tissues surrounding the infection. Severe complications requiring immediate hospitalization include Ludwig's angina, which is a combination of growing infection and cellulitis which closes the airway space causing suffocation in extreme cases. Also infection can spread down the tissue spaces to the mediastinum which has significant consequences on the vital organs such as the heart. Another complication, usually from upper teeth, is a risk of septicaemia (infection of the blood) from connecting into blood vessels, brain abscess (extremely rare), or meningitis (also rare).
Depending on the severity of the infection, the sufferer may feel only mildly ill, or may in extreme cases require hospital care. In 2011, Kyle Willis died from an untreated dental abscess in Cincinnati, as did fashion designer Hugo Boss, in 1948. 
See also 
- Neville, Brad W. [et al.] - 1st ed, 1995. Oral & Maxillofacial Pathology. pp.104-5.
- Gann, Carrie. "Man Dies From Toothache, Couldn't Afford Meds". ABC News. ABC News. Retrieved 20 January 2012.