Some quick water purification basics:
In order of decreasing size, biological threats in water include 1) cysts / parasites, 2) bacteria, and 3) viruses.
Cysts, being relatively large, are easy to physically filter out. Some can be tough and resistant to chemical sterilization.
Bacteria can be physically filtered out by the right kind of device; also many (but apparently not all) are vulnerable to chemicals.
Viruses, being very tiny, are tough to physically remove, but there are now products that can do so. Viruses are vulnerable to chemicals.
For more reading, visit
http://www.high-altitude-medicine.com/water.html - has lots of details on how to use various kinds of iodine and chlorine to maximum effect; very useful if you live in an area at risk of disasters. Note that this page is out-of-date as there are now devices that can physically remove viruses.
Used to be, there were two ways you could ensure to get *everything* biological out of water:
1) bring it to a boil
2) run it through a filter to get the cysts and bacteria, then disinfect (usually with iodine) to kill the viruses.
Nomenclature note: a 'filter' gets the cysts and bacteria, but not the viruses. A 'purifier' gets all three. At least that's how I remember the terms being used in the research I did online a couple years back. My memory may be faulty. Please do your own research to verify everything I'm saying, as your health is at stake here!
Sawyer has two grades of devices - one will remove cysts and bacteria, but not viruses; their higher grade will remove all three. So now there's a third option. Also, there are UV sterilizers that kill everything (need batteries; expensive), and some super-duper purifiers that yank the cysts and bacteria with a filter and use silver to kill the viruses (expensive). IIRC (from the Sawyer demo person who was at REI while I worked there), the higher grade has a slower flow rate, as you might imagine.
http://www.sawyer.com/products.htm
http://www.sawyer.com/biological.htm <-- page on their filter, the .1 micron one that gets cysts and bacteria
but not viruses. If you think viruses might be a threat (depends on geographic location), iodine IS required to kill them.
http://www.sawyer.com/viral.htm <-- page on their purifier, .02 micron, which gets everything.
Sawyer filters/purifiers have various configurations; think about how you want to use the system before you buy it (suck straight out of a bottle? Gravity-powered inline drip, which necessitates two containers? Option of doing both?). If I sound like I'm pushing Sawyer, it's because I'm budget-minded and Sawyer had the cheapest filters - with no cartridge replacement necessary - that I could find at the time.
If you want to pull out hazardous chemicals (like from ag runoff) or heavy metals (if you're in or downstream of an area that was mined), you'll need an activated carbon filter.
And, all this stuff works better (clogs a lot slower) if you filter out the larger stuff using a coffee filter, bandanna, or something similar first.