Leviticus 11:31
These are unclean to you among all that creep: whosoever doth touch them, when they be dead, shall be unclean until the even. These are unclean to you among all that creep: whosoever doth touch them, when they be dead, shall be unclean until the even.
Leviticus 11 is God's law given through Moses distinguishing clean from unclean, marking out meats pronounced unclean, prohibiting eating any unclean thing including unclean creeping insects, and identifying animals unfit for food including swine. It covers both which animals could be eaten and uncleanness from their carcasses, with restrictions on unclean animals and their carcasses standing alongside rules limiting which animals could be eaten. Specifically, touching the dead bodies of the listed unclean creeping creatures makes a person unclean until evening.
These restrictions were given as wise, just, and reasonable sanitary laws to preserve life and health, not as arbitrary commands. Health requires pure blood supported by simple, healthful food, and the prohibited animals were not the best articles of food but hurtful to the system. Swine's flesh and the flesh of certain other animals fills the blood and system with impurities and humors, pollutes the blood, and shortens life. In that warm climate swine's flesh often produced leprosy, with far more injurious influence there than in a colder climate, yet swine were never designed to be eaten under any circumstances but were useful as scavengers permitted to run free in a fruitful country to devour decaying substances on the ground that would poison the atmosphere, thus preserving health. If Israel had submitted to these prohibitions God would have removed everything injurious to health and suffered no sickness to be among them, but giving way to lust for what was forbidden brought disease.
God did not require straining all water used lest it contain the smallest insect that might be classed with the unclean animals. The Pharisees and other leaders went beyond what was specified by demanding that water be strained to remove even the smallest or tiny insect, expanding the rules into burdensome ceremonies and minute restrictions that kept people anxiously focused on distinguishing clean from unclean. Jesus condemned this focus on trivial external purity in contrast with their serious sins. The food appointed to Daniel and his companions at the king's table included such unclean meats, confronting the Hebrews with the prohibition against eating unclean meats. They had been taught that this class of food would not keep the mind or the physical structure in the best condition of health to do God's service.
They refused it to acquire knowledge to honor and glorify God, to perfect a Christian character, and to have a clear intellect to stand as representatives of the true religion amid the false religions of heathenism. Holding the will of God as the supreme law of life, they practiced temperance in eating and drinking so brain and muscle would not be enfeebled, choosing to keep mental powers clear and undimmed. Counting physical health a matter of the highest consideration, they refused to imperil physical and moral powers for indulgence of appetite. Seeing perils on every side, they met temptation with most decided efforts while trusting the rest with God.