There are various forms of Giardia that don't affect us, but G. Lamblia not only affects us, but also beavers ("Beaver fever"), domestic dogs, cats, cattle and sheep.
The old mountain men could have been affected with it, and attributed it to bad food, etc.
Also, the simple density levels of all the affected animals (including us) have risen exponentially into most areas that used to be fairly pristine. Where was the last place you went that you considered relative "wilderness" and didn't find human debris? And human waste?
Sue