Question

I don't know much about medicine, and I know even less aboutmicrobiology, but I understand that there are organisms in thelower gastrointestinal tract (and in feces) of a human, likeEscherichia coli, that can cause dangerous disease ifswallowed (or injected intravenously) by a human. So how did theyget to the lower gastrointestinal tract in the first place? Iassume either (or some combination of)

• its prevalence in feces is less than I'm led to believe, so myquestion as to how it got there doesn't arise;
• the percentage among cases of swallowed E. coli ofthose that lead to disease is low; or
• E. coli-caused disease is more common than I'm lead tobelieve.

Does anyone know which of these is true? Or is it somethingelse? (Perhaps they enter the gastrointestinal tract from below andnot via swallowing?)