germs.   biting spreads bacteria & can cause infection.    I know it sounds reasonable to bite back, but really all you are doing is using violence to stop violence and that has always worked in the history of the world (ya right!)    When my children bit,  I grabbed them hard, not to hurt them, looked them firmly in the face, shouted no and then isolated them from everyone (also known as time out)         Rest assured biting is a phase almost ALL children go through--some longer than others---and they will learn/grow out of it.