Tooth decay is a bacterial disease. The acids produced by the bacteria dissolves the enamel and cause destruction of the hard structures of the tooth. The cavities are not contagious, but the bacteria that cause it can spread from one mouth to another through used cups, food or kissing. Within the mouth cavities can “spread” from one tooth to another due to close contact. For e.g.The decay that happens between two teeth can be on the contacting surfaces of both the teeth.