Boiling and UV devices (For the PSK or BOB, I only know of the SteriPen; HERE ) are the only viable ways to be maximally safe from all waterborne micro-organisms.

It seems fine filtration can never be too fine; viruses WILL get through. Iodine can kill some pathogens, but usually takes quite a while, and can only merely reduce the numbers of other pathogen species.

Fine filtration AND iodine or chlorine dioxide can be effective, at least in theory. But if one organism is known to be resistant to iodine, it is logical to assume so might be another, perhaps one not yet discovered or not yet fully understood. Also, what we now know about certain microrganisms is not necessarily definitive and universal knowledge, as individual populations of microorganisms might for whatever reason (i.e. global warming; pollution having an unnatural nutritional or hormonal effect; genetic manipulation and release into the environment) mutate and experience accelerated evolutionary changes making them more chem-resistant, smaller, and/or more able to conform/deform during filtration. Survival of the fittest, Baby!

I am fond of my little bottle of Polar Pure as it is so self-contained and perpetual. Too bad it must be replaced.