Here is a funny story from Adam and my misadventures.  You can read the whole story in my travel writings section, but here is an excerpt:

From Isle Navarino we jumped a small (very very small) flight north eventually reaching Puerto Natales. From there we trekked the Torres del Paine-- a very popular 10 day circuit that is supposed to be one of the top 10 hikes in the world. It was amazing. The low point of that trip was when Adam and I were sure we had Giardia (one of the high ranking really nasty stomach illnesses). Once again, not our fault, really. Our guidebook said the water in patagonia is drinkable, but neglected to mention (we found out after we drank some untreated water) that beavers are a major carrier of Giardia. Tierra del Fuego is infested with beavers (to the tune of 75K of them having started from 25 mating pairs in the 1950s, giving a new meaning to busy beaver). Even worse, Giardia gestates for 1 to 2 weeks so you don't know if you have it. Bummer. As it ended up, what we really had come down with 8 days after drinking beaver water was an incredible case of stupidity. You see, we had bought this incredibly yummy jelly for our peanut-butter and jelly tortillas. Thanks to our fledgling Spanish, we didn't figure out until a day too late what "laxante" meant. We had actually made peanut butter and Laxative Jelly sandwiches. The good news is we dodged the giardia bullet and got our yearly supply of fiber (we had roughly 6-10 times the recommended dose of the 'jelly' in our lunch...) So far, that is the only stomach problem either of us have had, amazingly.