Dental decay is almost always a completely preventable disease. The key to prevention is removing plaque and bacteria before acid can eat away the tooth. Keeping the mouth and teeth clean with good oral hygiene habits can completely prevent this disease.
Brushing carefully at least twice a day, flossing at least daily, using fluoride toothpaste, limiting sugars and starches in the diet, and regular dental checkups can all help prevent tooth decay.
Cavities that are treated early usually do not have any long-term effects. If untreated, further destruction and invasion of the tooth occurs. This may progress to nerve irritation, causing pain and sensitivity of the tooth. A tooth abscess may form, which can rarely lead to a serious infection of the blood or heart damage. Even death is possible if treatment is never sought, though this is quite rare in the US.
This disease is not contagious and poses no risk to others.