How does a tooth become infected?

An abscessed (infected) tooth is caused by tooth decay. When the pulp (the soft tissue of a tooth that contains nerves, blood vessels and connective tissue) is diseased or injured and can't repair itself, it dies. The most common cause of pulp death is a cracked tooth or a deep cavity. Both of these problems can let germs (bacteria) enter the pulp. Germs can cause an infection inside the tooth. Left without treatment, pus builds up at the root tip, in the jawbone, forming a "pus-pocket" called an abscess. An abscess can cause damage to the bone around the teeth.