When bacteria (germs) enter a tooth through deep cavities, cracks, or flawed fillings, the inner part of the tooth (the pulp) can become infected. This is known as an abscessed tooth, which can cause pain and swelling. When left untreated an abscessed tooth can cause serious oral health problems.
If your dentist sees that you have an abscessed or internally-injured tooth, s/he may recommend the tooth be treated through a procedure called a root canal. During a root canal, the dentist removes the damaged nerve at the center of the tooth and seals the tooth with a filling or, in some cases, a crown.