According to the CDC giardia is found in all areas of the US and around the world so I would say that it is common - probably more so in 3rd world countries where sewer systems are lacking. Most likely people do get it in some form and may not get treated. The informaton provided says it could last 2 to 6 weeks. If I was sick that long I would definitely be seeing a doctor! I have never had it - I think. I have been ill with similar symptoms but never for as long as what the CDC provides. I have drank from streams before but after learning of giardia a while back I quit drinking from them unless I use my filter system. Here is a link if you wish to find out more about it - http://www.cdc.gov/parasites/giardia/gen_info/faqs.html I tried to find if you can build up an immunity to it but cannot find anything. My guess is no you cannot as these organisms attach themselves to your intestines and I don't think one bout with the disease creates the boundary that stops them from getting inside and attaching themselves to your intestines again.
I got about it about 15 years ago, rode it out but stayed sick as a dog for about three weeks. Wouldn't wish that on my worst enemy. Had no desire to eat anything knowing what the result would be. I've been drinking from the waters of Montana for 20 years total, no filter needed.
Let me start off by saying I am not a farmer.
I am a deer hunter and I do own a tractor and have all of the equipment - but that does not make you a farmer any more than standing in the garage makes you a car - well, you get the picture!
Having said that - and with most of my ineptitude fairly well hidden - I do think that buckwheat is one of the most under rated cover crops out there.
I say cover crop because I see mixed results as it relates to the deer eating the plants.