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.