Leviticus 11:32
And upon whatsoever any of them, when they are dead, doth fall, it shall be unclean; whether it be any vessel of wood, or raiment, or skin, or sack, whatsoever vessel it be, wherein any work is done, it must be put into water, and it shall be unclean until the even; so it shall be cleansed.
Leviticus 11 belongs to the law given through Moses that distinguished clean and unclean animals and forbade eating anything unclean. The distinction was not arbitrary but was given as sanitary law to teach principles of health and train in habits of cleanliness. Its purpose was to protect health and to preserve bodily and mental faculties through simple, healthful food.
Swine's flesh is an example of the forbidden food, prohibited because it is hurtful. It fills the blood and system with impurities and humors and shortens life. In a warm climate it often produced leprosy, with an influence more injurious there than in a colder climate. Swine were never designed to be eaten under any circumstances, but were made to devour decaying substances upon the ground that would poison the atmosphere and so preserve health. Other animals were also forbidden because they were not the best articles of food. Submission to these prohibitions would have removed everything injurious to health and left no sickness, while refusal brings injury through indulged lust.
Daniel's refusal of the king's food was obedience to this law, since the food appointed to him and his companions at the king's table included those unclean meats with wine and rich fare. That class of food would not keep mind and body in the best condition of health for God's service but would cloud the mind and enfeeble brain and muscle. Honoring God and standing as representatives of true religion amid the false religions of heathenism required a perfected character and a clear, undimmed intellect, with the will of God as the supreme law of life. Temperance in eating and drinking and refusal of unclean flesh protected physical health and physical and moral powers from being imperiled by indulgence of appetite. Resisting temptation in this required most decided effort and then trusting the rest with God.
Later religious leaders perverted these directions by extending them beyond what was required into unwarranted extremes and endless ceremonies and minute restrictions. They demanded that all water used be strained lest it contain the smallest or invisible insect classed with the unclean. This produced constant anxiety about whether water or food had been made unclean. Their added rules made external cleanliness a trivial concern beside actual sins.