of our microbes live around our cells, not inside them. Just think about your gut. It’s a long and heavily folded tube that, if spread out fully, would cover the surface of a football field. Swarming within that tube are trillions of bacteria. There’s just one layer of epithelial cells – the ones that line our organs – stopping them from penetrating the walls of the gut and reaching the blood vessels that could carry them to other parts of the body.
