It is true, but most of those bacteria are ones that we have developed a balance with (population sizes are kept in check by the body's environment and other li

It is true, but most of those bacteria are ones that we have developed a balance with (population sizes are kept in check by the body's environment and other life within it), so they do not pose a danger under normal circumstances. An open wound/near your mouth (maybe you've bitten your tongue accidentally) isn't much of a concern in that regard with your collection of bacteria (your immune system is aware of them and prepared), but if you bite someone else and break their skin you might transfer some cultures that your victim's immune system is not as prepared for. These populations are well suited to living in a human (that is why they are hanging around you), so easily take advantage of a human body with an immune response that takes time (due to lack of previous exposure) to ramp up a full controlling reaction.