Tooth decay develops due to three factors: sugar, acid-forming bacteria, and the susceptibility of a tooth.
 

Sugar provides food for acid-forming bacteria that then start wearing away tooth enamel.
 

The amount of sugar present in the mouth is important; however, the amount of time the sugar is present in the mouth contributes more to tooth decay.

Both natural and refined sugars can contribute tooth decay, and even "sugarless" foods contribute because they are made of substances that are easily converted back into sugars.

Milk, though it contains beneficial vitamins and minerals, including calcium and phosphorous, contains lactose, which is a sugar that is as equally threatening as sugar from other sources.

The best foods for healthy teeth and gums tend to be foods that require a lot of chewing. Chewing has several beneficial effects. Chewing stimulates saliva flow, which cleans the teeth, neutralizes acid, and thus helps prevent bacteria from thriving in the mouth. Foods that require a lot of chewing also massage the teeth and gums, helping to rid the mouth of plaque.

Raw foods and dairy products typically provide the most benefits for teeth and gums. Raw foods include fruits, vegetables, nuts, and whole grains. Raw foods provide fiber, protein, and beneficial nutrients such as calcium, magnesium, and phosphate. Dairy products include milk, cheese, and yogurt. Dairy products are helpful mainly because they contain calcium, which contributes to the development and strengthening of tooth enamel.

As always, be wary of sugars, and try to brush your teeth after every meal and floss at least once a day.