Add this to the growing list of reasons regular dental checkups and good oral hygiene are important: Cavities are contagious. Just like a cold or flu, you can "catch" cavities — or rather, the bacteria that contribute to caries — from kisses and shared spoons, for instance.

It's not new news, since studies dating back to the 1990s have clearly shown that specific bacteria, such as mutans streptococci (MS), can be passed back and forth, as can Actinobacillus actinomycetemcomitan, gingivitis and other cavity-promoting mouth crud. The cavities themselves are caused "primarily by bacteria that cling to teeth and feast on particles of food from your last meal. One of the byprodicts they create is acid, which destroys teeth," says Anahad O'Connor in a New York Times question-and-answer piece this week.

Sugar's not off the hook, either. It feeds the bacteria. So everything you've heard about sugar's negative effect on teeth — and the need to brush regularly and thoroughly — is true.

But anything that transmits any minute particle of saliva from one person's mouth to another, "like talking closely to the child," can get the tooth-destroying process started, notes Bradlee Dental Care, a dental practice in Alexandria, Virginia.

The Times article says that children and infants are particularly susceptible to the MS bacterium.

The U.S. National Library of Medicine in the National Institutes of Health lists two groundbreaking studies that changed thinking on the topic. The first, out of the University of Helsinki, Finland, in 1993, showed the transmission between spouses, looking at four particular couples to show what happened. Then, in 2007, researchers at the University of Queensland documented the transmission of the MS bacteria, which it called "cariogenic" to children from parents. And it noted that preventing cavities should include "timely control of colonization of the cariogenic bacteria in the mouths of young children."


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