I've been doing hardcore backpacking for about 20 years now. Only recently did I acquire a water purifying system, the Katadyn Micro-something or other. It's not too much to carry, weighs less than a pound. I only got it when my nephew got old enough to start going out on weekend hikes with me, didn't want to bring him home to his momma with giardia.

Before that, I carried iodine, but only used it maybe 10% of the time. I still believe that in the Rockies, if you drink from those little two foot wide streams coming off the cliffs, your odds of getting giardia are awfully low.

I have had giardia once for sure, another time maybe. I didn't go to a doctor either time and the symptoms, while unpleasant, weren't as bad as what I hear and read. The first time, in the Frank Church Wilderness in ID, me and another fellow on a trail crew drank out of a spring with bear crap floating in it. (Yes, bears do indeed crap in the woods. I have proof!) We were really, really thirsty. The diarrehia (?spelling) kicked in about 24 hours later, and gave me some anxious moments for a couple days, then went away. My buddy's symptoms lasted for a couple weeks, and he eventually wound up at the doctor's with a diagnosis of giardia and a prescription. He lost a few pounds.

The other time, I was solo in the Gila Wilderness in NM. I drank out of a spring full of cow pee. (Nobody ever asks, "Does a cow pee in the desert?" Why not?) Two weeks later, by which time I had migrated to the Weminuche Wilderness in CO, I began having intense stomach cramps and gas. This lasted for two or three days, and I believe may have been giardia, though I won't bet the farm on it.