It is a tooth that has become infected. When tooth decay advances deep into the tooth, it can cause destruction of the pulp, the core of the tooth. When this happens, the pulp gets infected and swollen and pus builds up near the jawbone. Left untreated, infected pulp can cause more serious problems, including damage to surrounding tissue and bone. Root canal treatment is the most common form of treatment. Root canal treatment involves removing the infected tissue, cleansing and sealing the canal of the tooth and restoring the tooth with a crown, or cap.