Get MSR Guardian at Amazon.
B&H Photo carries much of the MSR water filter product line including the MSR SweetWater Pump Microfilter(effective against bacteria and protozoa but not viruses), which is much less expensive than the 'Guardian'
Products like Katadyn MyBottle Water Purifier have a pore size of 30 nm which removes most viruses, but I’ll stick to my long-life MSR Guardian, pore size 20 nm because I can refill one or many bottles by pumping in a batch.
I use a water purifier in the field: a purifier takes out tiny stuff, like viruses. A water filter generally takes out only large stuff, like most bacteria.
The MSR Guardian water purifier has served me well, delivering over 100 liters this summer alone. I now use it as the only source of water while traveling in the mountains, up for two weeks at a time—nothing beats the water of the Sierra Nevada right out of the creek or lake (I also get fluid taste pleasure from GT’s Kombucha).
Now Science News reports in Microbial matter comes out of the dark that some bacteria are as tiny as viruses, at least one down to 20 microns, which is at the bottom of the 5 to 300 nanometer size range of viruses.
99% of all microbial species on Earth have yet to be discovered...
Some newly discovered organisms are so small that they barely qualify as bacteria at all. Jillian Banfield, a microbiologist at the University of California, Berkeley, has long studied the microorganisms in the groundwater pumped out of an aquifer in Rifle, Colo. To filter this water, she and her colleagues used a mesh with openings 0.2 micrometers wide — tiny enough that the water coming out the other side is considered bacteria-free. Out of curiosity, Banfield’s team decided to use next-generation sequencing to identify cells that might have slipped through. Sure enough, the water contained extremely minuscule sets of genes.
Bottom line: pure drinking water is not going to come out of a water filter. Only a purifier designed to take out viruses to 10 nm is also going to take out small bacteria. Whether or not this is a concern depends on many factors, but for surety, I’ll be sticking to a water purifier just for the virus aspect—the MSR Guardian should provide me with years of service and is rated for 10,000 liters.