There is so much to learn about bacteria, and how they affect your milk quality.
Bacteria are what causes hygiene-related milk quality grades. These tiny, single celled critters exist naturally all around us. Usually, we more or less ignore bacteria, discounting them as an irrelevant and theoretical menace in much the same way as we think about them at all. Which generally we don't.
However, if we take bacteria entirely for granted, there's every chance they will rise up from the foul crevices in which they lurk, and cause us untold trouble and misery. Again, comparison with the Alliance is irresistible.
Where does bacteria come from?
Everywhere. Dirt, water, the air, manure, animals, your skin, under the toe strap of your jandals, and any kind of organic matter including milk.