That depends partly on where you’ll be backpacking. In the U.S. and Canada, the most common pathogens to worry about in backcountry water sources are bacteria like salmonella, Campylobacter, shigella, and E. coli and protozoan cysts like Giardia lamblia and Cryptosporidium. These microorganisms can be trapped by a water filter.In parts of the world that are less developed, viruses like hepatitis A, norovirus, and rotavirus and similarly tiny organisms will slip right through a water filter, but a water purifier will kill them. Water purifiers use either a chemical like iodine or ultraviolet light to kill viruses and bacteria. A combination purifier/filter is recommended when traveling in less developed countries.