I wonder how many of you have had Giardia? It isn't fun. In a quest for an "Opening Day Golden Trout" Forty years ago, in knee deep snow I drank from a hole in the covered GIBBS STEAM about three miles from iced-over GIBBS LAKE.
Two days later Giardia struck me. Sorry no graphic details, but I was sicker than a dog.
Metronidazole (Flagyl), cured me quickly. It's an Rx drug.
A few summers later I discovered clear evidence of cattle grazing near the lake.
It's tough to beat the system I outlined earlier, and coupled with a filter recommended by the WHO and CDC the threat is eliminated.