Why do we have so many fucking boogers when we get a cold? I seriously have them dripping out of my nose right now. It's so gross!
For the same reason it's difficult to run up a down escalator: The faster the mucus leaks out of you, the harder it is for bacteria or viruses to get into you.
The outside world is a filthy pathogen-laden mess. Unless you're sick, the inside of your body is sterile—free from bacteria, virus, and fungus. Skin does it with layers on top of layers of cells, the outermost already dead and sealed into a thick sheet. Bacteria readily grow on this outer layer, but have little hope of blasting their way through the many layers to your innards.
Most of the surfaces where your insides meet the world are deep within your body, in the long tubes of the gut or the branching tree of the lungs. The lungs are a particular problem. At the very bottom of the airways, all that separates the outside world from your blood is one ultrathin layer of cells, a thin membrane, and one more cell wall. Basically nothing. With each breath, the air itself must be cleared of all pathogens before reaching the delicate bottoms of the lungs; so your body sterilizes each breath. How? With snot, and a lot of turning.
Every bit of your airways, from your mouth or nose to the alveolar sacs, is coated with mucus—a nice sticky coat. Let's say there's a hopeful little cold virus full of ambition, or a streptococcus bacteria ready for battle. One touch on any mucus-covered wall—in the nose, the back of the throat, the windpipe, the bronchi or bronchioles—and it's done, stuck in the muck. Snot is constantly swept upward to the top of the throat by hairlike cilia, and after reaching the top, the mucus—with the cold virus in it—is swallowed and digested. The odds of the cold virus making it safely to the Promised Land, where the blood is a short step away? Little to none. And even if it does make it that far, it's likely to be eaten by a macrophage or neutrophil, its host cell killed off by a T or natural killer cell.
When we do actually get sick—you can't expect this all to work perfectly—our bodies decide the winning plan is to speed all this up. "Oh, so you think you got us beat? More mucus, more sweeping upward!" Diarrhea is pretty much the same idea, for the gut. Be glad only your nose is dripping.
Pathogenically Yours, Science
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