Known to microbiologists as Giardia lamblia, this is the reason you don't drink the water when you go to a foreign locale, or camping. Giardia is a bacteria that will cause bloody stools and diarrhea, sometimes with a short fever. Locals don't get it, because they usually build up a tolerance to the bug.

It is not easily killed with Chlorine or Iodine treatments, so if you have to drink water you think is infected, use a filter or boil it. Or just drink it and take some Pepto-Bismol, because Giardia is rarely lethal.