No, not all bacteria in the mouth, or elsewhere in our bodies, are harmful. In fact, most bacteria found in our bodies are necessary and helpful. Only a few forms (less than 1% of all bacteria) cause disease; these are called pathogens. Bacterial oral infections are caused by these destructive forms of bacteria. Bacteria are harmful when they form toxins, when they invade tissues where bacteria do not belong, or when they do a combination of both.