I too have never chemically treated water on Isle Royale, but just to be clear, almost all filters do NOT remove viruses. One should always maintain an awareness of where their water to be filtered is coming from. If it is down stream from, or nearby to, any sort of human activity (e.g. outhouse) there is a chance of viral contaminants that filtering cannot be guaranteed to remove, hence the extra careful precautions given by the NPS and others to also chemically treat. Modern backpacking filters will remove the Hydatid Tapeworm eggs and almost all bacteria but not viruses.
Tapeworm eggs are approximately 25 to 50 microns. Bacteria are 0.2 to 10+ microns. NPS recommends to either boil or chemically treat plus filter down to a minimum of 0.4 microns. The CDC recommends a 0.1 micron filter to stop bacteria. Viruses can be as small as 0.005 microns. For comparison, a virus at 0.005 microns is only 40 times bigger than an Oxygen molecule at 0.00012 microns.
A lot of chemicals can be in the 0.001 micron range which is why filtering can sometimes still leave bad tastes (or even be unhealthy in agricultural areas). It is why when algae blooms are flaring up you should stay clear as noxious chemicals are produced that no portable filter or boiling can remove.
Summary: If you're going to drink water from a puddle full of scat, don't just filter it. ![Shocked :shock:](./images/smilies/icon_eek.gif)