This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
Some commercial microfilter incorporate a iodine resin stage to kill
viruses and bacteria, with out putting as much iodine in the water as
if it had been added to the raw water. A few products rely
exclusively on an iodine resin stage. Downside of these filters are
their fragile nature, dependency of effectiveness on flow rate and the
inability to identify when they need to be discarded. If you are
going to use one where the water is known to be contaminated with
viruses, then one of the better known brands such as the PUR or
Sweetwater Viraguard is recommended. More than one pass through the
filter may be necessary in cold weather.
Resins do have the advantage of producing less iodine in the water for
the same antimicrobal effect as for the most part, they only release
iodine when contacted by a microbe. The downside is that physical
contact between the microbe and the resin is