Why Do Our Teeth Decay?

Tooth decay occurs when food containing carbohydrates (sugars and starches) such as milk, sugar raisins, cakes or candy are frequently left on the teeth. Bacteria that live in the mouth thrive on these foods, producing acids as a result. Over a period of time, these acids destroy tooth enamel, resulting in tooth decay.
Tooth decay is caused by the bacteria found in dental plaque. In general, the longer dental plaque remains on a tooth's surface the more likely it will be able to form a cavity. The dental plaque that has accumulated on a tooth's surface is scrubbed off when a person brushes his teeth.
Some back teeth, especially molars, can be difficult for a person to clean because the grooves found on their chewing surface are deep and narrow. Even though the person brushes his teeth not all of the dental plaque that is present is cleansed off because the individual bristles of the toothbrush are simply too large to gain access into the depths of the tooth's grooves. Because some plaque has not been cleaned away, the tooth is at risk of decay.

The bacteria that cause cavities are living organisms. And that means they consume food and create waste products, just like we do.

As it happens, the waste products they created are very acidic (they have a pH of 4 and lower). And it's these compounds (primarily lactic acid) that cause tooth demineralization.

Bacteria and food can cause tooth decay. A clear, sticky substance called plaque is always forming on your teeth and gums. Plaque contains bacteria that feed on the sugars in the food you eat. As the bacteria feed, they make acids.