Filter systems have generally been constrained by their ability to eliminate only bacteria and Cryptosporidium (a microscopic, diarrhea-causing parasite), but not viruses. Purification systems get rid of viruses, but take longer to do so. What makes the MSR Guardian different is that it acts as both filter and purifier and is claimed to eliminate all biological threats you might find in even the dirtiest of water.
While there are other products out there that combine filtration and purification into a single unit, the Guardian does so without the use of chemicals, batteries, UV light, carbon or pre-filtering ... and it's self-cleaning.
The key is in the system's hollow-fiber structure made of what the company describes as advanced medical-grade fibers that are woven tight enough to block the smallest microbiological elements. MSR says the hollow fiber structure is undamaged by freezing temperatures and the Guardian can withstand drops from up to six feet onto concrete and up to 300 pounds of force.