When the nerve in a tooth dies, the infected tissue must be removed by either extracting the tooth or performing a root canal.  Removing the infection from the tooth and filling the canal where the nerve was removed is called a root canal (also known as endodontic treatment).
Reasons for a root canal:

• The nerve in a tooth can become infected for many reasons.
• A blow to the tooth can cause the nerve to die and become infected.

Ideally, the root canal should be done before the infection has a chance to get into the bone and before the area develops extreme swelling and pain.  Periodic dental examinations, and having your tooth checked when it first begins to hurt will usually help avoid severe complications.