Also, most filters can filter bacteria and cysts, but can't filter out viruses. Many say viruses are not an issue in the backcountry (but I still use purifying tablets), but if you're taking water from a suburban stream during a disaster, I'd definitely want to make sure I'm not ingesting whatever viruses the guy upstream deposited when he used the stream as a toilet.

Viruses do not survive in the open air well because they don't have a hybernation mechanism as opposed to bacteria