Ok, giardia, right. I've had it overseas before, couple of times. But deer, or any ruminant drinkin water & pooping in/around upstream water or a spring, can introduce it, not just cattle. Hence getting it out in the wild, ie the connection to wild unwashed watercress... No worries, likely never get it that way, but if you or anyone did, we fortunately have access to varied ways to kill giardia.
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"The flogging will continue until morale improves." ~ Unknown