In the past, if you had a tooth with a diseased nerve, you'd probably lose that tooth. Today, with a procedure called root canal treatment, your tooth can be saved. When a tooth is cracked or has a deep cavity, bacteria can enter the pulp tissue and germs can cause an infection inside the tooth. If left untreated, an abscess may form. If the infected tissue is not removed, pain and swelling can result. It is routine to process nowadays, not something to be feared.