Immunity may be possible according to Robert Rockwell. Speaking only of Giardia, being infected doesn't mean you get the disease. The feds estimate there are two million cases of giardiasis annually in the U.S. but something like 25 million people who are infected.
http://query.nytimes.com/gst/fullpage.ht...&st=cse

http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5401a2.htm

That said, most intestinial bugs that affect the general population aren't Giardia. There isn't really enough data to speculate on what backpackers get.

I recently read of a deal in Grand Teton National Park where through an entire season they invited anybody with diarrhea (not just backpackers) for a free test and treatment. They got 178 people of whom 8 percent had giardiasis, 28 percent had Campybato-something-er-other.. and 60+ percent had no detectable bug at all.
http://www.wemjournal.org/pdfserv/i0953-9859-003-03-0241.pdf