> Your intestinal bacteria will kill you (and themselves with you) if they escape into your circulatory system That's why you are constantly generating a mucus

> Your intestinal bacteria will kill you (and themselves with you) if they escape into your circulatory system That's why you are constantly generating a mucus layer with extreme diversity of sugar linkages, so that they are fed without one bacteria being able to take over the whole area (basically taming them). If instead you try to eradicate them with antibiotics (i.e. nuclear bombs) then you will inevitably breed ones that will kill you.