Because viruses are dicks.
They aren’t life — they’re closer to a protein or a vitamin than to a living thing, which means you can’t _kill_ them, only _totally obliterate_ them which is pretty hard to do with things smaller than a drop of water.
On top of that, they change all the time. They’re like tiny little biological computers that randomize their program to find out how to better fit their environment and this makes them really hard to obliterate.
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