Clean animals are those that are permitted to be eaten by Jewish or Islamic dietary law. Food permitted by Mosaic law is termed kosher. The Biblical definitions and examples of clean animals are provided in Deuteronomy 14 .
The most readily evident feature that distinguishes clean from unclean animals is their diet. The clean animals are all obligate herbivores (eating only plants), whereas the unclean animals are either carnivores or scavengers. The reasons for not allowing the consumption of the latter was only realized following the development of modern scientific investigation. Carnivores and scavengers frequently carry pathogens such as parasites, which are not found in herbivores. Some scavengers also accumulate toxins to levels that are dangerous to humans. Many contend that the Biblical definitions of clean and unclean is an example of the scientific foreknowledge that is found in the Bible.
- Main Article: Kosher
"Do not eat any detestable thing. These are the animals you may eat: the ox, the sheep, the goat, the deer, the gazelle, the roe deer, the wild goat, the ibex, the antelope and the mountain sheep. You may eat any animal that has a split hoof divided in two and that chews the cud. However, of those that chew the cud or that have a split hoof completely divided you may not eat the camel, the rabbit or the coney. Although they chew the cud, they do not have a split hoof; they are ceremonially unclean for you. The pig is also unclean; although it has a split hoof, it does not chew the cud. You are not to eat their meat or touch their carcasses. Of all the creatures living in the water, you may eat any that has fins and scales. But anything that does not have fins and scales you may not eat; for you it is unclean. You may eat any clean bird. But these you may not eat: the eagle, the vulture, the black vulture, the red kite, the black kite, any kind of falcon, any kind of raven, the horned owl, the screech owl, the gull, any kind of hawk, the little owl, the great owl, the white owl, the desert owl, the osprey, the cormorant, the stork, any kind of heron, the hoopoe and the bat. All flying insects that swarm are unclean to you; do not eat them. But any winged creature that is clean you may eat." - Deuteronomy 14:3-20
One group of clean animals are defined in Deuteronomy 14:6 as having a split hoof divided in two and that chews the cud. The text further clarifies that both the cloven hoof and cud chewing are requirements, and lists several animals (i.e. camel, rabbit, and pig), which have only one characteristic and are not permitted. Modern taxonomists place this group of clean animals in the order Artiodactyla (even-toed ungulates) and suborder Ruminantia (cud-chewing). The taxonomy of the Ruminants and a few examples are provided below.
- Suborder Ruminantia
Improperly cooked meats pose a health risk because of the numerous pathogenic organisms they might contain. Only in modern times have technologies improved to the point where we can be assured that meats have been sufficiently cooked to kill these pathogens. Even today in societies where these methods are unavailable parasites are a major problem.
Jane Cahill reported in Biblical Archaeological Review that the toilets of a Jewish household in Jerusalem were examined and no parasites or infectious agents were found. A similar study done regarding Egyptians systems revealed eggs from Schistosoma, Trichinella, wire worm and tapeworms, all found in pork.
Some unclean animals accumulate toxins to levels that are dangerous to humans. In 1953 Dr. David I. Macht, conducted toxicity tests on many different kinds of animals and fish, and concluded that the toxicity of Levitically "unclean" animals was higher than that of the "clean" animals, and that the correlation with the description in Leviticus was 100%.  Macht's study also matched the kosher classification performed by James W. Atz, PhD. Dr. Atz list of kosher and non-kosher animals was published by the Orthodox Union in Kosher Guide and in the Orthodox Union Kosher Consumer Directory. 
In addition to the aforementioned study testing kosher and non-kosher foods for toxicity levels, Macht had research indicating the harmful physiological effects of meat and milk combinations and he compared conventional animal slaughtering versus kosher slaughtering and determined that kosher slaughtering produced less toxic meat.
Animals on the Ark
- Main Article: Flood of Noah
Genesis 7:15 states that pairs of all creatures that have the breath of life in them came to Noah and entered the ark. Genesis 7:2-3 text also explicitly states that one pair of each unclean animal and seven pairs of each clean animal were brought on board to save them from the flood.
"Take with you seven pairs of every kind of clean animal, a male and its mate, and one pair of every kind of unclean animal, a male and its mate, and also seven pairs of every kind of bird, male and female, to keep their various kinds alive throughout the earth." Genesis 7:2-3
It is most helpful that the Bible gives these specific descriptions regarding the animals spared from the flood. In doing so the Bible helps us understand the actual evolutionary history of animals, because we know the maximum number of species that were reintroduced from each created kind following the flood. Arguably, each breeding pair could represent a separate species, therefore, only one species was present on the ark from each unclean kind of animal, and up to seven species from each clean kind. With this information in hand, we then know exactly how many gene alleles or species have developed for each kind.
Cleaned Through Prayer
It should be noted that many Christians believe they can eat the food that was declared unclean in the Old Testament if it is consecrated by prayer.
"For everything God created is good, and nothing is to be rejected if it is received with thanksgiving, because it is consecrated by the word of God and prayer." - 1_Timothy 4:4-5
Clean by Divine Decree
"He [Peter] became hungry and wanted something to eat, and while the meal was being prepared, he fell into a trance. He saw heaven opened and something like a large sheet being let down to earth by its four corners. It contained all kinds of four-footed animals, as well as reptiles of the earth and birds of the air. Then a voice told him, 'Get up, Peter. Kill and eat.' 'Surely not, Lord!' Peter replied. 'I have never eaten anything impure or unclean.' The voice spoke to him a second time, 'Do not call anything impure that God has made clean.' " - Acts 10:10-15
However, in Acts 10:28-29 Peter provides the meaning of the above vision as being a declaration or instruction from God for the Jews to take the gospel message to the Gentiles, which Mosaic law defined as unclean.
"He said to them: 'You are well aware that it is against our law for a Jew to associate with a Gentile or visit him. But God has shown me that I should not call any man impure or unclean. So when I was sent for, I came without raising any objection...'" - Acts 10:28-29
- It had to happen, Scientist Examines Ancient Bathrooms of Romans 586 B.C. by Jane Cahill and Peter Warnock. BAR May/June 1991