What causes cavities? "Everyone thinks they're just holes, but those holes don't happen on their own," Dr. Nowak says. Bacteria in the mouth (aka plaque) feed on certain carbs and make acid, which eats away at teeth. But how does plaque get into baby's mouth in the first place? It's usually passed from Mom or Dad. You might try a bottle or spoonful of food before giving the same object to baby. Some parents even "clean" a pacifier with their own mouth. Any of this back-and-forth equals an exchange of bacteria. So really, the best first step in preventing tooth decay is keeping your saliva to yourself. Make sure your mouth is clean and consider chewing gum with artificial sweetener xylitol; studies show this can help inhibit production and transmission of harmful bacteria.