All teeth have three main layers: the outer enamel layer, the inner dentin layer and the pulp. The pulp is the innermost layer within the roots of the tooth and contain the nerves and blood vessels that supply the tooth. The blood vessels provide nutrients to the tooth and the nerves provide sensation.
When the pulp is intact, the tooth is vital and healthy. When a tooth becomes decayed or fractured, the bacteria eat their way through the enamel and dentin layers until they reach the pulp tissue. Once here, they cause the pulp tissue to break down, causing destruction of the nerves and blood vessels within the pulp. As the tissue breaks down, an infectious process starts within the pulp and eventually the nerves and blood vessels die. Pressure from this infectious process builds up within the tooth, usually causing pain, and eventually an abscess develops at the root tip. Until this degraded tissue is removed from within the tooth, this painful, infectious process continues.