Here's some food for thought - the human body has upwards of ten times as many bacterial cells as it does people-cells. Bacteria are very small, so they don't take up as much space in our bodies as our human-cells, but our bacteria do play a role in everything from digestion to our immune systems. (The Wikipedia "human microbiome" page has some good basic info if you want to know more).
With all of this interaction between our bodies and all these bacteria that live with us, it turns out that healthy gut bacteria make for healthy people! And I found a lovely article this morning that summarizes the whole idea nicely:
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