In the Old Testament Times, the Law of Moses declared the following foods to be unclean and not to be eaten. If any of these were touched, the individual had to participate in some form of ceremonial cleansing.
Animals strangled, or dead by natural causes
Animals killed by other animals or birds of prey
Any animal that did not both chew the cud and divide the hoof
Animals classified as creeping things
Twenty or so types of birds mentioned in Lev. 11 and Deut. 14
Any water fish or animal that did not have both fins and scales
Any insect that had wings that did not also have four legs, with the two back legs for leaping
Anything offered in sacrifice to idols
All blood. (Any container which had had blood in it had to be purified.)
Any flesh cut from a live animal; and any discernible fat
The idea was that any animal was unclean if it bore the image of sin, or death, or of corruption, such as the larger land animals, carnivorous animals or birds, which lie in wait for living things and devour them (“the devil as a roaring lion…”). Also marsh birds and carrion birds which live on worms, carrion, and other impurities; all serpent like fishes and slimy shellfish, and small creeping things, except some kinds of locusts. “…because, partly, they recall the old serpent, partly they seek their food in all sorts of impurities, partly they crawl in the dust and represent corruption in the slimy character of their bodies” (Keil, Biblical Archaeology, II, 117 ff).
Anything dead was a source of ceremonial (religious) impurity.
The dead body of a human being, no matter how he had died, was unclean, as well as the building in which he lay, his clothing, any person who lived in the building or entered it. It was defiling to touch a body or a dead man’s bones or a grave. A person defiled in this manner also defiled everything he touched, or any other people he touched, until the evening of the day he was defiled.
Any animal carcass, clean or unclean, defiled anyone who touched it, until the evening, so that he was required to bathe himself and wash his clothes before being clean again.
Thus it was equally unclean to touch a dead animal of any kind as it was to touch an unclean animal which was alive.
There were eight kinds of small animals which spread their defiling influence to inanimate objects. These were weasels, mice, and six species of lizard. If any part of their carcass fell onto a cooking vessel, it was rendered unclean. If water had been contaminated by a dead animal, food which had been prepared in it was contaminated and could not be eaten. If such water had got some seed wet, the food which grew from that seed was polluted.
There were several types of defilement from bodily conditions and diseases
Leprosy rendered the person unclean until he was completely healed. The leper was required to tear his clothes, to bare his head, to put a covering on his upper lip, and to cry “Unclean” to everyone he met. He also had to isolate himself by living outside the camp or city. Houses infected with leprosy were examined by the priest, who, before entering, had all the contents of the house removed in order to prevent everything within from becoming unclean. If symptoms of leprosy were discovered, the house was closed for seven days, after which it was reexamined. If leprosy was still found, the affected stones were removed, along with scrapings of all walls, and the house was carefully replastered. Seven days after this, if there was still infection, the house was torn down and everything was taken outside the city to the place of unclean things.
Infected clothing was examined after seven days, and if still infected, it was burned. The purification rites for healed lepers is described in Lev. 8 and Lev. 14.
Any bodily discharge was unclean, and persons affected were considered unclean for a period lasting from seven to sixty-six days, depending upon the cause. Then they had to go through purification rites of cleansing. An unclean person was barred from touching anything holy or coming into the sanctuary.