The Steri-Pen is a water purfier that is manufactured by Hydro-Photon, Inc. You can visit their website (and see their research) at www.hydro-photon.com.
The Steri-Pen uses UV light to destroy microorganisms. A small UV bulb on the device is used to stir the water, and all the microorganisms in the water die. The Steri-Pen is reported to be effective against pretty much anything that could be in your water including (but not limited to): Cryptosporidium, Giardia and viruses.
