The human tooth is made up of both the hard and the soft structures. The hard or the calcified parts are the enamel, dentin and the cementum. The soft parts are the pulp (Nerves) and the periodontal ligament (gums).
The enamel is the outer most layer of the tooth. This is almost 99% calcium and is very strong. Unfortunately, the acids from the bacteria and the plaque that accumulates on the tooth dissolve the enamel and eventually cause decay. Once the enamel is destroyed, the softer dentin is destroyed much faster. Then the decay bacteria reach the inner pulp and the infection of the tooth sets in. This leads to pain in the tooth and swelling in the gums.