Five elephants

You carry more microbes in you this moment than all the people who ever lived. Those microbes are growing all the time. So try to imagine for a moment producing an elephant’s worth of microbes. I know it’s difficult, but the fact is that actually in your lifetime you will produce five elephants of microbes. You are basically a microbe factory.

-- from The Human Lake by Carl Zimmer

Gut bacteria may influence thoughts and behaviour notes the Neurophilosophy Blog, but Mike the Mad Biologist cautions:

It's de rigueur to begin a microbiome talk by stating that ninety percent of the cells in the human body are bacterial. Then there's a statement about how we should regard these microbes as another organ, like your heart or liver. And that's where things go off the rails.

The microbiome is not an organ. Those critters are not your 'friends.' They're not necessarily your 'enemies' either, but many of them function like a protection racket--get rid of them, and really bad news moves in.