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.
I have a system I use in Whitetail hunting during the rut that seems to work for me. When I make a mock scrape in the area I wish to hunt I will place a couple of 35mm
film canisters (film removed of course) in the scrape after I have cleared out all leaves and other debris. The film canisters are filled with cotton balls and saturated with doe in heat urine. This keeps the urine from soaking in the ground and can be refreshed on each trip if needed. I will also use the tarsal gland of the buck...