> Doesn't Saliva contain anti-bacterial properties?
Yes, in particular some IgA antibodies and lysozyme, but there are also plenty, and I mean **plenty** of bacteria in your mouth at any given point; we do, after all, still have to brush our teeth! In fact, animal (including human) bites should be treated lest they lead to sepsis. Those bacteria would _love_ all the magnificent nutrients in infant formula. Contact with an infant's mouth will introduce bacteria to the formula, as will exposure to air, contact with your hand, and anything else it experiences. As an example, in the past few years, there have been some cases (from China, I believe, someone correct me if I'm wrong) of formula contaminated with bacteria, which were more than happy to stick around.