The diagram shows a human tooth with an area of decay.

What is likely to have caused the decay?

Bacteria in the mouth feed on sugary, starchy foods and drinks (fruit, candy, bread, sodas, juice and milk). The bacteria convert these carbohydrates into acids.
Bacteria, acid, food and saliva mix to form plaque. This sticky substance coats the teeth.
Without proper brushing and flossing, acids in plaque dissolve tooth enamel, creating cavities, or holes, resulting in tooth decay.