How do mountain climbers handle contaminated water when they are in the wilderness?

They don't have filters. So what do they do with the water? How do they know it is safe to drink? Thanks to anyone that knows this and you will get points.How do mountain climbers handle contaminated water when they are in the wilderness?
There are a lot of small portable filtering systems that many people take into the wilderness.
If you boil water, and let the condensation collect on something, then move that condensation into another clean container (or straight to your mouth) the water is safe.How do mountain climbers handle contaminated water when they are in the wilderness?
Melting snow in a container. Bacteria tend to thrive in warm climactic conditions,so snow,being frozen,has most of it's contaminants eradicated by the low temperatures.