One of the things I forgot to mention in my original post is that a Water Filter is not considered a Water Purifier. A Water Purifier removes Viruses while a filter does not.

Also I forgot to mention that Viruses can be killed by Ultraviolet Light and that is the method that is used by some Purifiers.

Sorry for forgetting but I was very tired when I posted early this morning.

From what I have seen, MOST Filters do not have Activated Charcoal and Iodine Resin and do not kill Viruses or remove Chemical Contaminants.

The Katadyn Pocket Filter only has a Ceramic Filter Element, the one that has the Activated Charcoal is the Combi Filter.

Katadyn Combi Microfilter Specs:
Dimensions: 12" x 2.4" or 27 cm x 66 cm
Technology: 0.2 micron ceramic depth filter and activated carbon
Output: Approximately 1 quart or 1 liter per min
Cartridge Capacity: 13,000 gallons or 50,000 liters (ceramic)
Carbon Capacity: 105 gallons or 400 liters (carbon)
Weight: 21 oz or 580 grams

As you can see While the Ceramic Cartridge can filter up to 13,000 gallons, the Activated Charcoal part can only remove Contaminants from 105 gallons. The Charcoal can be replaced but it will mean carrying replacement Charcoal with you.

The Katadyn Filters do not remove Viruses. In order to remove them it is necessary to Chemically Treat the Water before using the Filter.

If I remember correctly one of the PUR Filters used to have an attachment that killed Viruses but the attachment only treated about 50 Gallons before needing to be replaced.

If you need to kill Viruses you need to use chemicals before or after you Filter the water or use one of the Ultraviolet Light Purifiers. Unless you use Reverse Osmosis or Boiling or better yet distillation.

By the way the MSR Sweetwater Purifier requires the Use of Chemicals to remove Viruses.

Inactivates 99.99% of waterborne viruses; eliminates more than 99.9999% of all waterborne bacteria and 99.9% of common protozoan parasites (such as giardia and cryptosporidia, as well as particulates, bad tastes, and odors)
System meets U.S. EPA requirements for microbiological water purifiers
Just pump, add five drops of the purifier solution per liter, and wait five minutes before consuming (for optimal protection)
System includes SweetWater® Microfilter, SweetWater® Purifier Solution, stainless-steel pre-filter, two color-coded silicone hoses, foam float, water bottle adapter, cleaning brush, and carry bag
Features a replaceable 0.2-micron labyrinth filter with a filter-eplacement safety indicator
A layer of granular activated carbon in the filter improves the taste of water and reduces chemical contaminants