Darwin, Wallace, and their peers were particularly fascinated by islands, and for good reason. Islands are where you go if you want to find life at its most outlandish, gaudy, and superlative. Their isolation, restricted boundaries, and constrained size allow evolution to go to town. The patterns of biology resolve into sharper focus more readily than they would do on the extensive, contiguous mainland. But an island doesn't have to be a land mass surrounded by water. To microbes, every host is effectively an island -- a world surrounded by void. My hand, reaching out and stroking Bab at San Diego Zoo, is like a raft, conveying species from a human-shaped island to a pangolin-shaped one. An adult being ravaged by cholera is like Guam being invaded by foreign snakes. No man is an island? Not so: we're all islands from a bacterium's point of view.

And another quote from later in the book:

When I drive through Chicago with Jack Gilbert, I experience the same dizzying shift in perspective. I see the city's microbial underbelly -- the rich seam of life that coats it, and moves through it on gusts of wind and currents of water and mobile bags of flesh. I see friends shaking hands, saying "how do you do", and exchanging living organisms. I see people walking down the street, ejecting clouds of themselves in their wake. I see the decisions through which we have inadvertently shaped the microbial world around us: the choice to build with concrete versus brick, the opening of a window, and the daily schedule to which a janitor now mops the floor. And I see, in the driver's seat, a guy who notices those rivers of microscopic life and is enthralled rather than repelled by them. He knows that microbes are mostly not to be feared or destroyed, but to be cherished, admired, and studied.

— Ed Yong, I Contain Multitudes: The Microbes Within Us and a Grander View of Life (2016)