Erika wrote an answer about MSR SweetWater Purifier System on July 5, 2010
The pore size of the filter is 0.2 microns (or 0.2 micrometers). Anything smaller than 0.2 microns will pass through the filter, which would be any virus (hence the name). Giardia and Cryptosporidium are protozoa, large enough to be filtered out, and most all bacteria will be filtered as well.
The active ingredient in Viral Stop is sodium hypochlorite, a common chlorine-based disinfectant. Your tap water at home is likely disinfected with this chemical, but probably was not filtered with a 0.2 micron filter first.
filter size: 0.2 microns
viruses: 0.05 to 0.1 microns
bacteria: ~0.5 to 50 microns
Giardia: 10 microns
Cryptosporidium: 5 microns
So use your best judgement. If you suspect the water has lots of bacteria (perhaps from a muddy pool), you could add it to be on the safe side.