Water Purification is an important item for hikers, a great companion for adventures where clean and drinkable water is hard to find.
Generally speaking, water purifiers are designed to remove or deactivate waterborne protozoa, bacteria and viruses from the water you are treating. Traditionally, backcountry water filters remove waterborne bacteria and protozoa, but have been unable to remove viruses, therefore, UV light, chemical treatments or boiling were required to deactivate viruses by scrambling their DNA or killing them. Today, however, new advancements in mechanical pump purifiers provide a convenient option to physically remove bacteria, protozoa and viruses quickly and easily.
If you are traveling to less-developed countries, where water treatment and sanitation infrastructure is poor and/or people don't practice good hygiene near water supplies, a water purifier (which removes or deactivates viruses) is considered the best option. Any waterborne viruses that are harmful to humans are transferred primarily through human waste. Therefore, where there are less humans, we assume the risk of viruses is also lower. Common viruses to be aware of include norovirus and hepatitis A. To combat these, a microfilter plus a purifying agent, like chemical tablets, is also a robust option.
It is important to remember that while chemical treatments, UV light and boiling will deactivate the microorganisms, they won't rid the water of particles. Particles in the water can impede the effectiveness of UV light and to a lesser degree, chemicals.
Mechanical pump purifiers should physically remove contaminants down to 0.02 microns and all purifiers should meet the U.S. EPA's Guide Standard and Protocol for Testing Microbiological Purifiers or the NSF protocol p.231. for removal of bacteria, protozoa and viruses.