If the pulp becomes infected, the infection may spread through the root canal system of the tooth. This may eventually lead to an abscess. An abscess is an inflamed area in which pus collects and can cause swelling of the tissues around the tooth. The symptoms of an abscess can range from a dull ache to severe pain and the tooth may be tender when you bite. If root canal treatment is not carried out, the infection will spread and the tooth may need to be taken out.