Several of my outdoor buddies contracted Giardia from surface water and or shallow spring water. I understand that none of them care to repeat the experience. I am also reading that Cryptosporidium (the organism most commonly isolated in HIV-positive patients presenting with diarrhea) is another Protozoa, like Giardia, with a hard protective outer layer making it resistant to many types of water treatment. Bacteria like E. coli and dysentery are larger pathogens that live in water, but like the Protozoa they are relatively easy to filter from water. Viruses, like Hepatitis A, may be less likely to occur but are not removed by common filtration systems. Be careful what you drink!

Based on the research I just did I don't think I will try filtering my water with my ceramic Spyderco stones though.