Generally tooth decay is not contagious. Tooth decay, also known as caries, is an oral disease in which the hard tissues of the teeth are progressively destroyed by a combination of factors, including bacterial infection. Cavities are formed by a combination of bacteria, diet, host, and time, so the mere presence of bacteria does not cause cavities, and cavities are not contagious. A sweet tooth can cause sugar to remain in hidden areas, and the bacteria in the mouth will react with the sugar to cause cavities, which are made worse by the host’s teeth being misaligned and lasting for a longer period of time. Cavities are usually found in people with poor eating habits and poor oral hygiene. People with cavities should use toothbrushes, floss, and rinse after meals to effectively remove plaque, adopt healthy eating habits such as controlling sugar intake, and have regular oral checkups. When tooth decay occurs, timely medical attention should be sought.