Most filters have both a very fine screen and some sort of activated charcoal. The very fine screen gets most of the bacteria and some viruses (they usually have some sort of iodine resin bed also to kill the rest of the viruses), and the activated charcoal adsorbs anything but water...It's not perfect, but pretty useful anyway.
Emergency Medicine - saving the world from themselves, one at a time. "Thou shalt not be a victim, thou shalt not be a perpetrator, but, above all, thou shalt not be a bystander." I make the ADL soil themselves. And that makes me very happy 
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