Some folks can probably live for a long time with a giardia colony on board without a lot of symptoms.
From what I've read, many people who are exposed to giardia won't show any symptoms and many of those who do show symptoms experience mild discomfort and recover without medical intervention. Apparently, it's actually a fairly small percentage of the populace that will have severe symptoms. Not that I want to play "giardia roulette" and find out which category I fall into.
Both my wife and I have had it, and we both had severe symptoms. It's not pleasant, like having constant stomach flu. Like Benjammin, we took Flagil (Metronidozal, I think), and it works like magic in a day or two. But you have to take the whole 10 day course.