For a pathogen to make us sick, it must overcome a lot. First it has to enter the body, bypassing natural barriers such as skin, mucus, cilia, and stomach acid. Then it needs to reproduce; some bacteria and parasites can do this virtually anywhere in the body, while viruses and some other pathogens can only do so from within a cell. And all the while, it must parry attacks from the body’s immune system.
So while we are constantly inundated by microbes, the number of microbes that enter our bodies is usually too low to get past our defenses. (A tiny
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