If bacteria get into the tooth pulp, also known as the pulp, they can cause inflammation of the roots. Root canal treatment is necessary to save the tooth.
The cause of an inflammation of the nerve in the root canal is usually deep Caries. But bacteria can also get inside the tooth if teeth have broken off because of an accident. In rare cases, a filling, which is close to the nerve, can cause similar inflammatory reactions in the nerve.
If the inflammation is left untreated, the bacteria can spread through the tip of the root into the surrounding tissue. The classic, painful "thick cheek" with an accumulation of pus can appear as a symptom. A painless death of the tooth nerve is also possible. The local chronic inflammation can put stress on the whole body. An X-ray examination is necessary for diagnosis.
(Source - Prodente.de)