Boiling does not always work, it is time and altitude sensitive. Boiling water at sea level is at least 212 degrees F, at 8 or 9 thousand feet it is hard to get over 160 degrees F. The proper boiling time is also critical for a lot of organisms; Giardia being one of the hardest to kill and it is one tough little mutha, and it will kill you.
Giardia is pretty common here in the Pacific Northwest.
There are all kinds of streams and rivers that flow as clean and clear as you could wish for, little bacterial (pathogenic) contamination.
But giardia is in almost all of them. And it is invisible to the naked eye.
Another thing that people might want to think about is a cheapo microscope. You’d be amazed at the things you might see even in supposedly clean water! I once looked at some of the water in my dogs dish about 8 hours after I filled it. What a zoo!!
Giardia is big though (relatively speaking) and is able to be trapped in microfilters.
Don’t believe it can survive UV disinfection.
You could always use a pressure cooker to get the water up to a higher temperature.