Other bacteria in the air settling on it, multiplying, and generating waste products. Why don’t those bacteria make use sick when we just breathe them in? Because we are breathing them in at very low concentrations, so our immune system/gastrointestinal tract can deal with them. It’s a bit like having an enemy army attacking yours one by one vs all at the same time. If you’re fighting the enemy one at a time, you have a much better chance to beat them than if they swarm you.
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