This is a small, woodland brook near my home in Vermont’s Green Mountains. It is clear and cold as can be. Other than occasional hunters, there is no human activity upstream of this point. But would you drink from it?

I did for many years. On a hot day it was wonderful to lie on my stomach here and gulp that cool water. I was never sickened by it. But eventually I learned about all the bad stuff that might be there, unseen. And so I quit drinking untreated brook water.

Specifically, Giardia, an intestinal protozoan parasite that by all accounts can make your life totally miserable. Both beaver and deer can host this creature, which means it has the potential to be found in almost any backcountry brook or pond. You can’t see these without a microscope, so any water–no matter how appealing it might look–offers some risk.

On the other hand, Giardia (and other bacterial and viral nasties) are not found in all mountain streams. At higher elevations and absent any major human or farm animal activity, there’s a fairly good chance the water is safe to drink. Various medical authorities disagree on the relative level of risk. REI, the backcountry hiking and camping outfitter, has an excellent article about this on its website, which you can read here.

Increasing awareness of Giardia has fueled a whole industry of portable water-purification systems–water filters, tablets, and more–that can be found in every mountain shop and outdoor-retail catalog. I’ve used many of them. Pumping a hand-held water filter is a genuine nuisance.

So it’s hot. You’re hunting or fishing the backcountry, and you’re thirsty. There’s a wonderful-looking cold brook. Do you drink from it? Or not?