Well look at you. Aren’t you a masterpiece of biological evolution? You are a big, ambulatory, autonomous human being. You are separate from the insects on the ground, the birds in the air, and the steer in your burger. As a human, you envision yourself as lord of your surroundings, a unique animal risen and separate from other “lower” forms of life. Look at those clothes! And those thumbs! You are a whole and special individual, a single, isolated member of a species that has dominated and partitioned itself off from “nature” through years of rugged conquest and ingenuity. You could be very smug about all of that.
Except you’d be wrong.
You are not alone. While it is no secret that humans share their bodies with a bunch of microscopic, and smallish macroscopic, guests, the scope of their pervasiveness and impact on our lives is not commonly understood. The role of microorganisms and other parasites in the human experience extends far beyond a bit of armpit odor, bad breath, head lice, and dandruff.
In the perspective of a bacterium, the outside of the human body is an endless substrate, strewn with a ridiculous amount of nutrients and minerals, set on a lumbering factory that just churns out more goodies, making human skin an attractive place to settle down and colonize. The inside of humans is even better! It’s warm and moist and if you’re in the right organ system, new nutrients get delivered right to you! Human bodies are to bacteria as a mystical candy house in the woods is to Hansel and Gretel. It is only by the graces of the immune system provided, non-stop, by vigilant human cells that you and I aren’t eaten from inside out…and also from the outside in…by our plentiful tenants.