After all this talk about filters, and viruses, I saw this article on the norovirus today:

"New Norovirus Strain Rips Through The U.S."

So I started looking for natural treatments for a virus. That led me to this article on sauerkraut, which I think is pretty interesting. This especially got my attention:

"These probiotics kill off bad pathogens in the intestines such as bacteria, yeast and viruses."

Makes me wonder if putting a little juice from sauerkraut would sanitize water. Seems like it'd be worth looking into.