I have commented on water purification on previous posts. My real question is how do you know if it works? You use any method, and it works. You do not know if it would have been OK without purification. I sure would like to see a real scientific study done. I just think we have gone way overkill on this issue.
There is no question that in most places abroad or in really yucky water anywhere, purification is needed.
I honestly doubt the need to purify water in many of our U.S. mountainous regions. (PS, I am a groundwater geologist and water resource engineer and have attended many conferences on water treatment, so I have a fair knowledge of water quality issues.) Water in the mountains is continuously naturally being recycled and purified.
There is a lot more to this issue than purify or not. Improperly cleaned filters can actually ADD microbes. You carry the thing in your dirty old pack all day! Touch the purified water bottle with your filfthy hands - more germs. With chemical methods, water temperature is critical. I suspect a lot of supposedly "purified water" really is not fully purified but was OK to begin with so you just think your technology is the reason you do not get sick.
Everyone has an immune system that can handle a certain amount of bugs. Everyone's tolerance is different. I take chlorine and iodine tabs and only use them occassionally and have hiked a lot- mountains, coasts and deserts in the continental US for 40 years and never have had problems. I am very careful of my water sources. My immune system seems to be quite hardy. Maybe my eating all that dirt when I was a kid helped!