Some relevant comments I've excerpted from a nationally recognized expert on giardasis as related to backpacking, from his article at

http://www.yosemite.org/naturenotes/Giardia.htm

I haven't edited out the footnotes, but you can study the citations in the link to his original article.

Understand that giardasis is among various fairly common diarrheal illnesses, and the vast majority of cases are not contracted from backcountry water sources.
Here are selected excerpts :


If you have symptoms it may not be giardiasis 1, 7, 13, 16, [32]

Many people claim that they “got it” on a particular trip into the wilderness. Yet upon questioning, they usually report that the presence of Giardia was not confirmed in the laboratory. (Only 8 percent of persons with a diarrheal illness in this country seek medical care.22) ..

,,,,The diarrhea being blamed on Giardia from that climbing trip a week ago may instead be due to some spoiled food eaten last night or Campylobacter in undercooked chicken four days ago. Or because the incubation period is usually from one to four weeks, even if it is giardiasis the uncertainty range indicates that the culprits could have been ingested anytime during a full three weeks worth of meals and beverages. People in high-risk groups for Giardia, such as family members of children in day care centers or promiscuous male homosexuals, have even more possible sources to consider. To indict a particular stream or lake under such circumstances, without being able to at least verify that cysts are indeed there at all, is illogical at best.

...Almost always, giardiasis goes away without treatment 1, 6, 7, 13, 14, 15, 30, [34] ...

The US Food and Drug Administration, observing that giardiasis is more prevalent in children than adults, suggests that many individuals seem to have a lasting immunity after infection. [35] Furthermore, citizens of cities and countries where the parasite is numerous clearly seem to have few if any problems with their own water, which also points to an acquired immunity. So there is a possible bright side to contracting the disease.
....
Rare individuals not only do not spontaneously rid themselves of the organisms but instead develop serious symptoms of malabsorption, weight loss, ulcer-like stomach pain, and other chronic disturbances. Fortunately, this occurs in fewer than 1 percent of those with infestations. These unlucky people need medical treatment.