A gum boil is a small opening in the gum that drains pus or blood if pressure is applied to it. The adjacent tooth is often painful on chewing but sometimes pain may not be present. The most common cause of gum boil is dental abscess, which resulted from deep caries and bacteria infecting the pulp and the surrounding gum. The body’s immune response to fight the infection produces pus. The build up of pus leads to tissue destruction and formation of the gum boil to drain the pus. Once you notice a gum boil over a tooth, a visit to the dentist or endodontist is advisable. The dentist will investigate the cause and drain the pus by starting root canal treatment.