Tooth Decay also known as Dental Caries is the most prevalent human disease, characterized by a localized destruction of calcified tissue, initiated on the tooth surface by decalcification of the tooth enamel, followed by enzymatic lysis of organic structures, leading to cavity formation. If left uncheck, penetrates the enamel and dentin and may reach the pulp. Carious lesions develop most commonly in areas beneath dental plaque in which stagnation occurs. Several theories on etiology have been proposed.
Acidogenic Theory, whereby acid produced by bacteria caused decalcification and softening of the residue
Proteolytic Theory,whereby microorganism destroy enamel protein,
Proteolysis-Chelation Theory, whereby keratolytic microorganism caused formation of chelates which , in turn, cause decalcification.
(Source: Jablonski, S (1982)Illustrated Dictionary of Dentistry. W. B. Saunders Co. P146)