Leviticus 11:28
And he that beareth the carcase of them shall wash his clothes, and be unclean until the even: they are unclean unto you. And he that beareth the carcase of them shall wash his clothes, and be unclean until the even: they are unclean unto you.
Leviticus 11 is the Mosaic health and sanitary law distinguishing clean creatures from unclean ones. It forbade eating anything unclean, including swine's flesh and the flesh of certain other animals. Animals that died naturally or were not properly drained of blood were also prohibited as food. The dietary restrictions were intended to limit the harm of eating flesh.
Contact with such carcasses was defiling, so that he who carried one must wash his clothes and remain unclean until evening. The chapter is also invoked for the food prohibition without elaborating this carrying-and-washing rule. The restrictions were given to preserve life and health. Simple, healthful food kept the blood pure and protected mind and body, while unclean food fills and pollutes the blood with impurities and shortens life.
Daniel and his companions honored this law by choosing food that avoided those unclean meats, so as to preserve their health and clear mental powers for God's service. The law was perverted by burdensome human traditions and unwarranted extremes. The Pharisees extended God's restrictions with excessive requirements, such as requiring all drinking water to be strained lest the smallest insect in it be classed with the unclean animals, which the law itself did not call for. Jesus exposed this perversion by contrasting such trivial exactions with the magnitude of actual sins.