If the pulp (nerve) tissue dies; it invariably becomes infected by bacteria because it is warm, moist and is a source of bacterial nutrients. These bacteria can come from the decay or surprisingly enough by bacteria that occasionally can be found in the blood stream. Once the pulp chamber is infected, the bacteria multiply and the bacteria and the toxins they produce exit the tooth tissues at the tip of the root and cause an inflammatory response. White blood cells called into the area engulf the bacteria and kill them or produce enzymes and antibodies to kill them. The mass of dead white cells and bacteria is called an abscess.
This abscess starts locally but migrates through the tissues trying to drain into the mouth. This causes the gum to swell and eventually burst releasing pus. Before that happens; it can be exquisitely painful. The quicker you see the dentist, the better.
On severe cases; this abscess enters the tissues around the face, tongue, throat or neck and cause a red swelling which is very painful as well. This can end up in a serious stage and may need to be hospitalized. Don’t put it off and act urgently!