(I) Some people have especially active, decaycausing bacteria in their mouth. (II) A parent may pass these bacteria to a child, presumably by kissing. (III) However, one must understand that not all tooth pain is caused by cavities. (IV) The bacteria flourish in the child's mouth after the first teeth come in and can then cause cavities. (V) So a tendency toward tooth decay that runs in families doesn't necessarily reflect poor oral hygiene or bad eating habits.