When the dental pulp (nerve) inside the tooth dies then the space inside the roots (root canals) will inevitably become infected. The microbes in the root canals will leak out the end of the root causing an abscess. This may occur rapidly within days, or it may be slow taking a year or longer for an abscess to appear.
The symptoms from a tooth like this may vary from severe pain, swelling of the face, systemic illness including a fever, difficulty in swallowing, to no symptoms at all other than a draining point beside the tooth. Dental infections are a major cause of hospitalisation and need to be thorough investigation and treatment.
Sometimes a tooth has already received a root filling but the inside of tooth has not been cleaned adequately or the root filling and/or the restoration is leaking. These teeth can also develop a dental infection.