Boiling works.

An example- Some years ago I was a fortunate member of a cave exploration team that spent a month in southern China, mapping and exploring some incredible passages. A registered EMT at the time, I was responsible for first aid and medical issues. I figured we would have lots of diarrhea, etc., especially given the waste disposal/fertilization system in use, all in close association with the water supply.

All the water that we consumed was boiled by our Chinese chefs. We had no, absolutely none, cases of runs during our month long stay, eating and drinking the local cuisine. My vast stock of anti-D pills went entirely unused.

Burncycle makes an excellent point about water sources in a disaster. The further downstream you are, the greater the chances for contamination, especially from items like petroleum, pesticides, and the like.

Most of my experience has been in back country, high up, and i have never experienced ill effects from drinking untreated water, a common practice of mine 40-50 years ago. That is historical data, and one might be a bit more cautious now.

Boiling works, and with a good iso-butane stove, gives you just enough time to smell the roses. I often cool down the boiled water by immersing the container in the nearby mountain stream - just takes a few minutes - or have a nice cup of tea right there.