Well, the answer to your question depends on your wanted range of Bacteria. I assume you are referring/restricting to Mutualistic or Commensalistic Bacterium that are known to reside naturally in the Human Body specifically? You will have to clarify. If you are referring to any organism in the Domain Bacteria altogether, then the answer would simply be that any pathogenic Bacteria (Any Bacteria that causes disease) is harmful in the wrong place (The "wrong place" being the sector of the human body in which it causes the given disease).