What Switchable Suzie said about opportunistic pathological (bad for you) bacteria is so important, that it deserves to be reinforced:
The most common lower bowel pathogen may well be clostridium difficile, which kills luminal intestinal cells by producing a poison, "cytotoxin B". This causes the bowel to slough off parts of the lumen wall surface, producing a type of diarrhea called "pseudo membrane" diarrhea, so called, because part of the runny stool looks like thin sheets of membrane, but are not. It does not tend to go away by itself.
C. difficile is a tough bacterium, needing a large amount of competing, benign bacteria to prevent it from growing to dangerous levels.
Disclaimer: I am not a doctor. The succeeding story is not medical advice. It is an anecdote, not a suggestion.
One should always see a doctor when sick, because your condition may not be the same as someone else's, and your body certainly is not.
When that (very, very rarely) happens to me, I use a probiotic (an off-the-shelf cocktail of harmless, competing bacteria). I have used tablets of 2 billion organisms and of 15 billion. I usually get well with 3 tables of 15 billion organisms, 8 hours apart, for two days. I only get temporary help from the two billion tablets.
Don't misinterpret the foregoing as medical advice. It is an anecdote, not a suggestion. I am not a doctor.