I don't know much about medicine, and I know even less about microbiology, but I understand that there are organisms in the lower gastrointestinal tract (and in feces) of a human, like Escherichia coli, that can cause dangerous disease if swallowed (or injected intravenously) by a human. So how did they get to the lower gastrointestinal tract in the first place? I assume either (or some combination of)
- its prevalence in feces is less than I'm led to believe, so my question as to how it got there doesn't arise;
- the percentage among cases of swallowed E. coli of those that lead to disease is low; or
- E. coli-caused disease is more common than I'm lead to believe.
Does anyone know which of these is true? Or is it something else? (Perhaps they enter the gastrointestinal tract from below and not via swallowing?)