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Dental caries characterizes by localized dissolution of the hard tissues of tooth due to the production of acids from bacterial metabolism in dental plaque. It has been known as the major cause of dental pain and tooth loss in populations all over the world (Fejerskov and Kidd 2003) throughout the history of mankind and is still a major public health problem worldwide (Selwitz et al. 2007). In most industrialized countries, dental caries affects 60-90% of school-aged children (Petersen 2005a). Dental caries experience of 12-year-old children, based on WHO Global Oral Health Data Bank, is high in Americas (DMFT> 3.5) and in the Oral Health Care – Prosthodontics, Periodontology, Biology, Research and Systemic Conditions 208 Region (DMFT> 2.5) (Petersen 2003a). Looking at the DMFT data from countries show that there are several countries with fewer than three DMFT but high Sic Index [the Significant Caries (SiC) Index: mean DMFT of the one-third of the group having the highest DMFT in a population) (Bratthall 2000)] values illustrating the hidden caries burden for children (Nishi et al 2002). According to the U.S. Surgeon General’s report (U.S. Public Health Service 2000), dental caries is the single most common chronic childhood disease in the United States. About 20% of US population in all age groups has been found to have untreated dental caries (National Center for Health Statistics, 2011). Two recent studies in UK revealed that more than one-third of 5- and 11-year-old children in Great Britain had evidence of caries experience in dentine (Pitts et al. 2006, 2007). In most developing countries, the level of dental caries is tending to rise due to excessive sugar consumption and inadequate exposure to fluoride (Petersen et al. 2005). In adults the situation is more worrisome as dental caries affects almost 100% of the population in the majority of countries (Petersen 2005b). Dental caries is not restricted to children and young adults. The elderly constitute a particular risk population especially regarding root caries (Petersen and Yamamoto 2005). It is worth mentioning, therefore, that dental caries has not been eradicated, but just controlled to a certain degree (Marthaler 2004, Petersen et al. 2005). Dental caries, in its advanced stages, is associated with considerable pain, anxiety and impaired social functioning. Caries-related tooth loss causes eating disability, reduces self esteem and impair quality of life (Chen et al. 1997). In addition to its burden on individual, dental caries imposes a significant economical burden on communities. Tooth-related diseases have been considered as the forth most expensive to treat in industrialized countries (Petersen 2004). More than 51 million lost school hours per year has been recorded in the USA, due to dental-related illnesses which means approximately 3.1 days per year for 5-17-year-old children (National Center for Health Statistics 1996). There are calculations which show that the budget for restoring the permanent dentition of the child population of low-income nations with amalgam would exceed the available resources for the provision of an essential public health care package for the children of 15 to 29 low-income countries (Robert and Sheiham 2002). 3. Determinants of dental caries Dental caries has been frequently acknowledged to be a multifactorial disease meaning that it has many causes (Baelum and Fejerscov 2003). Each single factor with a probable role in caries development presents a possible cause. Caries occurrence in different individuals, however, is not due to the operation of all of these possible causes. Different people may have different sets of causes for caries development, which are defined as sufficient causes. Each of these sets, in turn, may consist of many single causes (Rothman 1986). Caries will not develop until all the component causes of a sufficient cause have accumulated (Baelum and Fejerscov 2003). Tooth (the host), microorganism (the agent) and diet (the environment) (Fejerscov 1997) are major necessary factors in the process of