Leviticus 11:25

And whosoever beareth ought of the carcase of them shall wash his clothes, and be unclean until the even.

Leviticus 11 is the law given through Moses that pronounced certain meats unclean, distinguished between clean and unclean, and forbade the eating of anything unclean, including the ritual requirement that whoever carries the carcass of an unclean animal must wash his clothes and be unclean until evening. These restrictions were not arbitrary, but were given as sanitary laws to preserve life and health by keeping the blood pure through simple, healthful food, with enforced distinctions between clean and unclean.

Swine's flesh and the flesh of certain other animals were forbidden because they fill the blood with impurities and shorten life. Swine's flesh in particular was expressly forbidden because it is hurtful, filling the system with humors and in that warm climate often producing leprosy, being far more injurious there than in a colder climate, and swine were never designed to be eaten under any circumstances, while the other forbidden animals were not the best articles of food. Swine were useful as scavengers, permitted to run free in a fruitful country to devour decaying substances on the ground that would otherwise poison the atmosphere, and in that way served to preserve health. If Israel had submitted to these wise prohibitions, God would have removed everything injurious to health and allowed no sickness among them, but because they would not submit He gave them what they lusted for.

The law was perverted by unwarranted extremes beyond what God required, when the people were required to strain all drinking water lest the smallest insect, even unseen by the eye, be classed with the unclean animals. This turned it into trivial outward exactness that Jesus rebuked as straining at a gnat and swallowing a camel while neglecting great inward sins. This same law of unclean meats is the background for Daniel and his companions' refusal of the king's food.