Caused by certain types of bacteria (Streptococci and Lactobacilli) that live in your mouth. When they attach themselves to the teeth and multiply in dental plaque, they can cause damage. These bacteria feed on what you eat, especially sugars and cooked starch. About 5 minutes after you eat, or drink, the bacteria begin producing acids as a by-product of their digesting your food. Those acids can penetrate into the enamel of the tooth and dissolve some of the minerals like calcium and phosphate. If the acid attacks are infrequent and of short duration, the saliva can help repair the damage by neutralizing the acids and supplying minerals and fluoride that can replace those lost from the tooth. However if:

1. Your mouth is dry

2. You have many of these bacteria

3. You snack frequently

Then the tooth mineral lost by attacks of acid is too much, and cannot be repaired. This is the start of tooth decay and leads to cavities.