Ksane, I know it's a bit more complicated that than but I wanted to head off the "since my water is clean, it can't be e coli" thoughts. My mom had an indoor cat die from e coli and the only thing her and the vet could come up with on how the cat got it was from catching and eating a mouse. E coli has the nasty habit of being very simple or highly complicated, it all depends upon the strain and the organism (animal) that picks it up. For example, people became extremely ill from e coli after drinking apple juice made from apples collected in a orchard frequented by deer (google Odwalla e coli apple juice). Sensitivity and resistance is a factor in becoming sick from any given strain.