Well, good question. Actually, none of the top three water treatments are guaranteed to give you pure water. Filtering (Sawyer fo example), Chemicals(AquaMira or Iodine for example) and Radiation (Steripen for example) are all statistically based. You can kill 99.9999% (or render them impotent) but the one that gets through has a finite chance of infecting you. Even boiling can leave heavy metals and some fertilizers in the water.
All is not lost. I normally use two half liter bottles, filled and sterilized in the morning. If it will be a dry day, I will add some to a spare bladder. If I know I will cross several water sources, I may not carry any. That is entirely terrain dependent.
I usually have a Steripen in the pack, and, AquaMira. AquaMira is reserved for overnights, mornings. I use UV during the day, as needed. I also boil water in the morning and evening. Filters are ALWAYS heavier than AM drops for mid length trips, ie up to about 3 weeks. I repackage AM into small bottles, good for about three weeks. They weigh about an ounce. The Adventurer Opti weighs about 4-5oz, depending. In 7 years, I have never had one fail in the field. (I managed to kill one at home, though. They replaced it, no problem. Previous to the Opti, it was just the Adventuror which I never had trouble with, but lost on a hike.)For longer than three weeks, a UL filter (like the Sawyer) would be my choice.
With the advent of the newer UL filters, the weight and flow rates have changed a lot. So, I no longer can say it is any lighter than the Steripen, even at 3.5oz. But it saves about one bottle of water. As does the Sawyer, if done correctly. Or, about 1 pound. So, either pays for itself in overall weight savings.
I would note that simply changing the mineral content of water can change your digestive system, giving you diahrea or leaving you constipated. This is NOT a disease or parisite.