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I've always been told to wash my hands so that I won't get food poisoning - there are bad bacteria in the feces (such as e coli), which lay spores, and if the spores get into the intestine, they start reproducing, causing food poisoning.
But if these bacteria are in my feces, they'd be in my intestine! So what would be the harm for one to ingest a few more bacteria, when there are already hundreds where they came from?
In your large intestine, some pathogenic bacteria, like E. coli, can be present as part of "normal intestinal flora" without causing any problems, because other (mostly nonpathogenic) microbes limit its growth.
When you do not wash your hands after having a bowel movement, some E. coli bacteria can be present in the stool on your hands, so you can transfer them to the food where they can freely multiplicate (in much larger numbers than in your large intestine) and produce toxins. When you ingest the contaminated food, the toxins can affect your small intestine and trigger diarrhea.
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