I recommend a water filter and a purifier. Hows that? A filter to get out sediment, bad tastes, and little creatures; and a purifier to take care of nasty viruses and other tiny things a filter cant get.
The Miniworks Filter
Myself, Ive always been partial to the MSR Miniworks ($89) for a filter. Its easy to use and maintain, its water-pump" handle is convenient and comfortable, and its very effective against giardia and other protozoa. It also removes chemical tastes and other pollutants. On the high end, Katadyns Pocket Filter ($289) is made of extremely tough materials for long use (stainless steel and aluminum) and also does a very good job of filtering.
To take care of viruses, treat filtered water with Sweetwater Purifier Solution ($10 for two ounces). Basically high-quality household bleach, the purifier kills whatever the filter misses. Its billed as approved only for Sweetwater filters (same company as MSRMSR Sweetwater Microfilter, $79), but cmon, it works the same no matter what filter is used. Katadyn, meanwhile, sells chlorine dioxide tablets that achieve the same end ($12 for 30).
Whether you have filtered or unfiltered water, you can also add solution from the MSR Miox purifier ($140), which uses salt and a battery-powered electrical charge to create a solution that inactivates" bacteria, protozoa, and viruses. Light and easy to use.
In either case, learn good filter management. Try to draw from water that is as clean as possiblethat will help the filter last longer and make it easier to pump. Also follow the manufacturers recommendations for how and how often to clean the filter. And remember that it can take hours for some viruses to die even with a treatment, so read directions carefully.