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. The Guardian meets US military testing standard–NSF Protocol P248 and is able to fit on most standard water bottles. It can filter and purify up to 2.5 liters of water per minute and can produce over 10,000 liters of clean water before it’s no longer useful.The self-cleaning feature sees ten percent of the water that's pumped through the system fed back through the filter through a separate line to flush it clean. So no more hassling with scrubbing cartridges or backflushing to ensure both a steady and clean flow.