Water filters...the subject that has many opinions...and few facts. Lots of anecdotal evidence and marketing hype.
Most filters do a reasonable job of removing bacteria, crypto, and giardia. So for many folks who are backpacking, a filter or chemical treatment (chlorine dioxide, for example) will suffice. You should not have to use both, or filter AND boil. One treatment method will do.
Chemical treatments - iodine works for bacteria and viruses, but may struggle to take care of crypto. Chlorine dioxide will take care of all of them, but may require extended (up to 4 hour) contact time to do the job. UV works, but I would not trust them with water that is even a little cloudy (fans of UV - carefully read the instructions and cautions - the definition of contact time and water clarity makes it clear that they do not make any guarantees if the water is cloudy).
So pick the limitations you would rather live with. All treatment methods work, they all have drawbacks.
Common sense rules. Are you in a remote area, with few people? Are there beavers, or high mammal traffic in the water? Or are you in a heavily used area? Downstream from farms or popular campgrounds (especially family campgrounds)? The answer determines how worried you should be. If in a remote area, viruses are probably not a concern. But in a heavily used area, even in the US (yes, in the US) then you might want to worry about viruses. Water anywhere could, but may not, contain harmful bacteria, giardia, and cryptosporidia in concentrations high enough to make you sick.
The problem is, you can't tell by looking if there are things in the water that can make you sick. And some sources may be fine today, but not tomorrow. It depends on what's upstream, and if there has been recent heavy rainfall. And on top of that, your individual immune system health plays a very large role in how many of the particular critter you have to ingest to actually get sick. Have you been getting enough sleep? Been stressed out at work lately? So you may be fine, but your buddy may get Montezuma's Revenge from the same water source. Or vice versa.
Bear in mind that getting ill from your camping buddy's poor hygiene is far more likely than getting ill from drinking contaminated water. The fact is, most backcountry waters (remote or relatively so) do not contain enough bacteria or giardia to make you sick. I have seen a number of articles and test results in recent years that bear this out. In fact, if you look around you can find things like the reports from the Sierra, which show that most backcountry waters in the Sierra have fewer Giaradia than the municipal water in San Francisco. I'd have to look to find them again, but they are out there on the web.
Years ago ('70s), I went on a number of backpacking trips in the Adirondacks. We never treated the water - we just dipped our Sierra cups. We never got sick. Maybe we were just lucky. Or maybe (and this might be the case) we had been exposed enough that we developed an immunity.
Pore size is a marketing tool, nothing more. None of the filters sold today, with the possible exception of the MSR Hyperflow, actually have holes the size they claim. What they have are filters with an equivalent pore size of .2 or .3 microns. The bacteria are not often removed by being physically larger than the pores in the filters. Rather, they adsorb onto the surface of the filter. They must travel a tortuous path, and the quality of the filter determines the likelihood of the bacteria being adsorbed. This also means that no filter is 100% effective at removing bacteria. But they are effective enough most of the time.
Another half-truth is that filters will not remove viruses. A quality filter (MSR Miniworks, Katadyn Pocket, for example) will remove in the high 90s percent of the viruses in the water. They do this because viruses are normally not free floating, but are attached to particles. They are also adsorbed onto the filter media, just like the bacteria, despite the fact that they are considerably smaller than the "pores" in the filter. Filters do not, however, remove a high enough percentage to qualify as a purifier according to EPA standards, which is 99.99%. Katadyn even says this on their web site, and MSR and Sweetwater (before it was bought by Cascade Designs) did a few years ago. Look at the Katadyn Pocket filter, which has been used all over the world, to good effect, for something like 60 years or more. It is a simple, high quality filter.
http://www.katadyn.com/en/technical-support/filter-support/faq/
Sometimes you will see an article that compares filters and how easy they are to use. What you will not see is a comparison test of how the filters actually perform when challenged by real bacteria when filtering real field water. The exposure history (adsorption history) of the filter plays a large role in how effective it is after lots of use. The last such test I saw was in 1997, done by MSR. It was illuminating. They bought filters from most manufacturers and hired people to sit and filter raw river water with a visibility of about 12 inches. That is fairly murky water, much more so than I like to filter. They filtered water until the filter reached end of life according to the manufacturers' instructions, whether it be a certain number of cleanings, or a minimum wall thickness of the element, etc. They then tested the filters with water that had 480 million bacteria/liter (typo in the report linked below). That may seem like a lot, but the bacterial count goes up substantially after heavy rains, you'd be surprised at how much is in the water. The bacteria used was Brevimundas Diminuta, about 0.3 microns (the EPA test protocol calls for the use of Klebsiella Terrigena, about 0.7 micron). It turns out that the top two were simple filters, not the so-called purifiers! And the top one was not made by MSR, in case you were wondering (the second was, but they no longer sell it). See the tables on pages 16 and 18 in this document. The two tables show the filters in the same order, if you want to know. Do the math - start with 20 million bacteria/liter (a quite reasonable number) and see how many are left after filtering.
http://bschool.washington.edu/gbc/documents/MarathonCase.pdf
Be patient, it is a large file. And remember, the test was done 13 years ago, so the filter manufacturers may have changed their products since then.
The bottom line is that most filters remove enough for most people. If you have a weak immune system, or you are treating water from a questionable source, you may need to do more. In remote areas, you might not need to treat the water at all. As for me, I normally filter, and use hand sanitizer, and don't let my campmates use my cooking gear without washing their hands, or me washing things after they use them. You may choose another method over filters for your own reasons.
