What is the difference between a water purifier and a water filter?
Water filters work by forcing water through a mechanical device, while purifiers often use chemical or electrostatic processes in addition to mechanical filtration.
Whether or not you should use a filter or a purifier depends primarily on the water source quality you are drawing water from. Because most water filters, like the Katadyn Hiker, will filter particles in your water as small as .2 microns, they are more than adequate to handle water born bacteria and protozoa. However, a water purifier, like the MSR MIOX, are required to kill particles as small as .004 microns which include viruses. Typically, viruses are only a problem when drawing water from areas where humans are nearby.
It is certainly possible to drink water from a mountain stream without becoming ill, but do you want to take the chance? Many portable water filtration options are available to ensure safe hydration. Some water bottles have their own water bottle filters, or you can choose a separate filtering device for water disinfection.