You're out on a long run and your bottle goes dry. You start experiencing the early signs of dehydration. You nowhere near a drinking fountain or a store. What do you do?
I've run into this scenario many times over the years... and always choose the same course of action- I scavenge for water anywhere I can. That has included drinking from streams, lakes, a water hose in some random person's yard, and making a funnel out of a leaf during a rain storm.
There's always an inherent danger in drinking water from questionable sources. The water may contain organisms that can make you sick, like giardia or dysentery. The water could also be contaminated with poisons which could kill you. It's always a gamble.
In an emergency situation, I look for water that contains some form of life. If a water does not have any signs of life (fish, plants, etc.), odds are pretty good that it's undrinkable. Next, I make a filter out of a bandana and a water bottle by placing the bandana over the mouth of the bottle and attaching it with rubber bands (I keep a few rubber bands wrapped around all m water bottles). When the bottle is submerged, the bandana acts as a filter.
It's not nearly as effective as boiling the water, using a commercial filter, or chemically purifying the water, but I rarely if ever have the tools required for elaborate purification. The homemade filter will likely trap most of the harmful stuff and is better than drinking straight from the questionable source.