Plus a little bacteria helps keep the immune system in shape.
A little giardia, however, would not be good for keeping anything in shape. Well actually it may be good for your stomach muscles as they will get a week long workout and not in a good way
JPD, I used to do the same thing as you. There was a spring by my house that everyone would drink from and fill up water bottles to bring home. I do not know of anyone ever getting sick from that water. Eventually it was shut down when it tested positive for urine. What I quickly learned is that just because it has been safe to drink in the past does not mean it is safe now. It just takes one time for something to contaminate it and boy will you be sorry.