A tooth infection is when bacteria enter the nerve and blood supply in the inner part of a tooth, infecting the tooth and causing an abscess. An abscess is a pocket of pus at the base of the root caused by bacteria accumulation. A tooth infection typically causes pain, swelling and tenderness in the area and requires treatment. If not treated, the infection will grow and potentially cause serious effects such as the jaw and even the brain. Tooth infections occur due to untreated cavities, old restorative work and trauma. It is essential to see your dentist immediately when you think you may have a tooth infection, and your dentist will likely recommend an antibiotic. The type of antibiotic recommended depends on the type of bacteria and the type of infection.