1. During the midle ages Salt was a very expensive good used for medicinal purposes. Because of this, spillage was to be avoided at all costs..
2. The idea that it brings bad luck comes probably from the belief that Judas spilt salt during the last supper. Throwing spilt salt over the left shoulder is linked to its medicinal use..
3. If it could not be administered, the next best thing was to throw it into the eye of the evil spirits that brought sickness upon us. These spirits were thought to lurk behind your shoulder, waiting for an opportunity to strike..