Is it possible to drink straight from backcountry streams and never become ill? Yes.
Is it possible to drive down a large city's main boulevard, ignore a few red lights and never have a fender-bender? Yes.
Is either practice worth the accompanying risks? In my opinion, no.

Here's a brief overview to help clean up the confusion surrounding which type of water-treatment system is right for you. Free-flowing mountain streams, for all their beauty and clarity, are not always the fountains of purity we imagine them to be. Backcountry water sources — crystal-clear rivers, lakes and streams — sometimes harbor microscopic pathogens (disease-causing agents) that are tough to pronounce, difficult to spell and, for many people, awful to ingest. Giardia lamblia. Cryptosporidium. Campylobacter jejuni. Hepatitis A. All are members of an invisible fluvial zoo that may be present in pristine-looking backcountry water.
How do they get there? When water becomes tainted by animal or human feces. What impact could such microbes have? They can leave you reeling with diarrhea, abdominal cramps, nausea, weight loss and fatigue. How long might these symptoms last? Between 4 and 6 weeks. Maybe longer. Ugh. Portable water filters and purifiers both operate on the same mechanical principle. Using a hand pump and intake hose, both slurp up "raw" water from a lake or stream and force it through an internal element (a filtering "medium"). This medium traps suspended elements — from fine sediment to invisible microorganisms—before dispensing clean water into a container of your choice.
What's the Difference? Some definitions: Water filter—A microbiological device that removes bacteria (e.g., Campylobacter jejuni) and protozoan cysts (Giardia lamblia, cryptosporidium) from contaminated water. Water purifier—A microbiological device that removes bacteria, protozoan cysts and viruses (e.g., hepatitis A) from contaminated water. Viruses are infinitesimal organisms too tiny to be trapped by a filter. Devices identified as "purifiers" usually cause water to interact with iodine (often in the form of iodine resins), which can render viruses inactive. Another purifier uses a positive electrostatic charge in its filter medium to capture viruses. Viruses: * may exist in water wherever there is a reasonable chance of human fecal contamination; * are believed to be less prevalent in North American wilderness water sources than protozoan cysts or bacteria, but may be a greater threat in less developed countries. Over time, filters have proven that they reliably protect wilderness travelers from the most common waterborne pathogens found in the North American backcountry: giardia and cryptosporidium. Still, purifiers and their antiviral feature offer an elevated level of security.
There are a few basics that I look for in a water filter. The Katadyn is a mechanical, ceramic micro filter and uses no chemicals (healthier). Filters down to 0.2 microns (will remove all giardia, fungi, parasites, etc.) Built in pump produces about a quart a minute. Zippered carrying pouch included. This filter has a few little extras that really make it nice, such as the pre-filter for extended life of the main filter and the clip for attaching the ouput hose to your water bottle. I love this filter and even though it is pretty darn expensive I think that it is well worth the money.