The Chinese know that when one has a wound, shellfish, mollusks, fish without scale, and other exotic / bush meats (literally called 'wild taste' in Chinese) should not be consumed or else the wound will worsen. The Chinese also believe some eels or catfish are lethal mutants (literally called "bone-melting dragon" by Cantonese) so one should not eat them to avoid the risk. They deem such meats as noxious.

Some reasons
Let us take a quick look on a few examples of why some meats are not that good.

Predatory animals often prey on the weakest (sometimes diseased) in herds. Fish scales reflect light to repel parasites and also protect the fish from scratch that is exposed to disease. In meat insufficiently bled, putrefaction happens rapidly even frozen.

Scavengers are for cleaning Planet Earth (so that it is more suitable for humans to live), not for consumption. Some even practice coprophagia (i.e. eating feces). Their function is simply like worms that degrade carcasses. The older the scavengers, the more harmful their meat becomes due to accumulated toxins. In Indian Ocean tsunami 2004, squids were seen attached on floating carcasses.
The risk of eating raw molluscan shellfish
Dangerous Lung Worms Found in People Who Eat Raw Crayfish

Pork consumption can cause cirrhosis (research by Nanji and French 1985), trichinosis, pork tapeworm parasitic disease, cysticercosis, brucellosis and Hepatitis E.
Philippines to slaughter 6,000 pigs for Ebola Reston

Many say eating dogs, dolphins and monkeys is inhumane because those animals are intelligent. But nobody says eating pigs is heartless although pigs are also intelligent. Pigs can be trained for law enforcement to sniff out narcotics.

Salmonellosis can be contracted by ingesting ground meat (mixed with fat and blood), reptiles or rodents.

Angiostrongylus cantonensis is a parasite in rats and snails that can infect human.

Blubber contains naturally occurring pollutants. Since whales place high on the food chain, they are bound to consume large amounts of pollutants. High levels of mercury have been found in the blubber of seals of the Canadian Arctic.
Japan warned on 'contaminated' blubber
Chemical Compounds Found In Whale Blubber Are From Natural Sources, Not Industrial Contamination
Trichinosis from walrus meat

AIDS, SARS and Ebola outbreak were due to consumption of exotic meats. Biohazard. It looks like unclean meats can also spread deadly disease besides immoral / unclean activities.

Consuming undercooked frog or freshwater eel causes Gnathostomiasis.

In 1953 David Macht made a study on phytopharmacology. He saw that extracts from meat of unclean mammals, birds, and fish inhibited the growth of lupin seedlings more than clean ones. His report is available for download at the bottom of this article.

In Dark Ages, the Jews suffered less (due to strict diet of not eating unclean meat) from plagues and were thought to spike the wells.

The Bible
The Bible advises about 'clean' and 'unclean' meat. Even if not going for the option of unclean animal, humans still have a lot more varieties of food than animals have. But humans have piggish appetite. Babies put anything into mouth. Adults outgrow little of this childish tendency. Adam and Eve ate the forbidden fruit. Israelites asked for meat (Numbers 11:4) after their hunger in the wilderness were fulfilled by manna. Some Christians say food does not affect faith so we can eat anything we like. Yes, they are right. It is not forbidden now to eat unclean meat but anyway we still should take care of our health.

Unclean meat guidelines are stated in Leviticus 11 and Deuteronomy 14. For health sake we should avoid the types of meat that are declared as unclean in the Bible. All carnivores except fish (some fish although with scales and fins, are carnivorous) are considered unclean. Milk and eggs of unclean animal are to be avoided too. This article, based on the characteristics that the Bible describes, also includes examples of animal that are not mentioned in the Bible.

On land
1) Chewing the cud but not having hoof, for example camel, hare (and other lagomorph). Note that chewing the cud means chewing what have been partially digested, including but not limited to ruminating.
2) Having split hoof but not chewing the cud, for example swine. Swine is a scavenger. Swine depicts spiritual hypocrisy because it has split hooves and white meat as if the pork is good meat for food but actually it is bad for health.
3) Walking on paws. The Bible does not give example for this one but by definition the examples should be canine, feline, and bear.
4) Moving about with belly near to the ground: weasel (and other mustelidae), mouse (and other rodent), tortoise (and other reptile), land snail (and other pulmonata), mole (and other talpidae). Other examples by definition are frog (and other amphibian), pangolin, armadillo (and other xenarthra).
5) Not chewing the cud and not having hoof or the hoof is not split / cloven: any others that do not fall into category 1 to 4 such as (by definition) horse (and other equine), monkey (and other primate), rhinoceros, elephant, tapir, flightless bird (e.g. ratite).

Can live in water (able to feed and breed in water; not in Noah's ark)
1) Having fin but not having scale. The Bible does not give example for this one. By definition the examples should be catfish, cartilaginous fish (e.g. shark, ray), swordfish, seahorse, lamprey, marine mammals including cetacean (e.g. whale), sirenian (e.g. manatee).
2) Having scale but not having fin. By definition the example should be sea snake.
3) Not having fin and not having scale. By definition the examples should be crustacean (e.g. shrimp, prawn, crab including land crab, barnacle, astacidea such as crayfish), mollusk (e.g. squid, scallop, gilled snail), echinoderm (starfish, sea cucumber), cnidaria (jelly fish). These are scavengers and detritivores.
Not many people are familiar with fish name. The best way is to look at the fish. If the fish has many small thin overlapping plates on the skin that must be removed / scaled before the fish can be eaten, then it is a 'clean' fish.

Can fly
1) birds that can prey on vertebrates, for example eagle, kite, owl, hawk, cuckoo, cormorant, swan, pelican, stork, heron, lapwing.
2) scavengers / carrion eaters, for example vulture, raven.
3) bat

Bugs
All. But those with taller rear legs specially for leaping, for example locust, can be eaten.

Others
1) All animals found dead (Exodus 22:31, Ezekiel 44:31).
2) All animals not properly slaughtered / bled (Acts 15:20, Acts 15:29) except fish.
3) Blood of all animal (Genesis 9:4, Leviticus 3:17, 7:27, 17:13, Deuteronomy 12:16, 12:23, 15:23, 1 Samuel 14:33, Acts 15:20, Acts 15:29).
4) Fat of all animals (Leviticus 3:17, 7:23).

Repetitious supporting biblical verses (not there for nothing)
Genesis 7: "(KJV - King James Version) ² Of every clean beast thou shalt take to thee by sevens, the male and his female; and of beasts that are not clean by two, the male and his female."
By the way, Noah seemed to be a Tarzan because he could command all animals to go into the ark, ha ha. Noah was not a Jew. Jews came out of Abraham later. Noah would be given the permission to eat meat after the flood. So it is inaccurate if someone says the concept of unclean animal is meant for Jews only.

Leviticus 20: "(KJV) ²⁵ Ye shall therefore put difference between clean beasts and unclean, and between unclean fowls and clean; and ye shall not make your souls abominable by beast, or by fowl, or by any manner of living thing that creepeth on the ground, which I have separated from you as unclean."

Judges 13: "(KJV) ⁷ But he said unto me, Behold, thou shalt conceive, and bear a son; and now drink no wine nor strong drink, neither eat any unclean thing: for the child shall be a Nazarite to God from the womb to the day of his death."
The same is repeated in verse 7 and 14.

Isaiah 65: "(KJV) ⁴ Which remain among the graves, and lodge in the monuments, which eat swine's flesh, and broth of abominable things is in their vessels;"

Isaiah 66: "(KJV) ¹⁷ They that sanctify themselves, and purify themselves in the gardens behind one tree in the midst, eating swine's flesh, and the abomination, and the mouse, shall be consumed together, saith the Lord."

Ezekiel 4: "(KJV) ¹⁴ Then said I, Ah Lord God! Behold, my soul hath not been polluted: for from my youth up even till now have I not eaten of that which dieth of itself, or is torn in pieces; neither came there abominable flesh into my mouth."

Ezekiel 22: "(KJV) ²⁶ Her priests have violates my law, and have profaned mine holy things: they have put no difference between the holy and profane, neither have they shewed difference between the unclean and the clean, and have hid their eyes from my sabbaths, and I am profaned among them."

Hosea 9: "(KJV) ³ They shall not dwell in the Lord's land; but Ephraim shall return to Egypt, and they shall eat unclean things in Assyria."
This chapter talks about punishments. Eating unclean meat can be a punishment because of its miserable outcome on health.

Matthew 3: "(KJV) ⁴ And the same John had his raiment of camel's hair, and a leathern girdle about his loins; and his meat was locusts and wild honey."
John the Baptist did not eat unclean meat.

Matthew 13: "(KJV) ⁴⁷ Again the kingdom of heaven is like unto a net, that was cast into the sea, and gathered of every kind: ⁴⁸ Which, when it was full, they drew to shore, and sat down, and gathered good into vessels, but cast the bad away."
What bad can be caught in the fishing net?

Matthew 23: "(KJV) ²³ Woe unto you, scribes and Pharisees, hypocrites! For ye pay tithe of mint and anise and cummin, and have omitted the weightier matters of the law, judgment, mercy, and faith: these ought ye to have done, and not to leave the other undone. ²⁴ Ye blind guides, which strain at a gnat, and swallow a camel."
Jesus condemned the Pharisees of judging small bad / unclean things too much but they are doing big bad / unclean things.

Mark 5: "(KJV) ¹³ And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand;) and were choked in the sea."
Later in verse 17 the people in the pig-keeping region asked Jesus to leave because they were afraid their pigs would also be killed.

Revelation 18: "(KJV) ² And he cried mightily with a strong voice, saying, Babylon the great is fallen, is fallen, and is become the habitation of devils, and the hold of every foul spirit, and a cage of every unclean and hateful bird."

Verses mistakenly used to oppose the advice of clean and unclean meat
Genesis 9: "(KJV) ³ Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things."
Background: After the Great Flood, Noah came out of the ark together with clean and unclean animals that Noah brought into the ark. By common sense, that does not mean every animal and plant can be eaten. Nobody will eat tapeworm or poison ivy. Seven pairs of each clean animal type were brought into the ark in comparison to one pair for the unclean, because clean animals would become human food after the flood.

Mark 7: "(KJV) ¹⁸ And he saith unto them, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot defile him;"
Background: The Pharisees saw Jesus' disciples eating food with unwashed hands. From the context of the whole chapter, the subject is about unwashed hands, not about unclean meat. The Pharisees watched everything Jesus Christ and disciples said and did. Perhaps the Pharisees were the earliest paparazzi in history. They tried to find trouble to the extreme extent that even a minute matter like unwashed hands was condemned. But there were no accusation of Jesus Christ and disciples eating unclean meat at all, simply because Jesus Christ and disciples did not eat unclean meat. Do you think the Pharisees would easily let go if they did see Jesus Christ and disciples eating unclean meat? Modern versions of Bible add "(In saying this, Jesus declared all foods 'clean')" in Mark 7:19. That is really not good.

Luke 10: "(KJV) ⁸ And into whatsoever city ye enter, and they receive you, eat such things as are set before you:"
Background: Jesus was sending people to cities to preach the gospel. From the whole context, this verse does not talk about clean or unclean meat. It is impropriate to just quote one verse without looking at the whole context. Again, Jesus did mean food when He said food. Unclean meat is not food.

Acts 10: "(KJV) ¹² Wherein were all manner of fourfooted beasts of the earth, and wild beasts, and creeping things, and fowls of the air. ¹³ And there came a voice to him, Rise, Peter; kill, and eat. ¹⁴ But Peter said, Not so, Lord; for I have never eaten any thing that is common or unclean. ¹⁵ And the voice spake unto him again the second time, What God hath cleansed, that call not thou common."
Background: Peter was a Jew and he preached gospel only to Jews. If we read the context of the whole chapter, we will know that the subject is about Jew and non-Jew, not clean and unclean meat. God wanted him to preach gospels to all people, not only Jews.

Romans 14: "(KJV) ¹⁴ I know, and am persuaded by the Lord Jesus, that there is nothing unclean of itself: but to him that esteemeth any thing to be unclean, to him it is unclean.", " ¹⁷ For the kingdom of God is not meat and drink; but righteousness, and peace, and joy in the Holy Ghost.", " ²⁰ For meat destroy not the work of God. All things indeed are pure; but it is evil for that man who eateth with offence. ²¹ It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak."
Background: Some Christians in Rome were weak in faith that they were convinced to just eat vegetables, abstaining even from clean meat. And they also did not drink wine.

1 Corinthians 10: "(KJV) ²⁵ Whatsoever is sold in the shambles, that eat, asking no question for conscience sake: ²⁶ For the earth is the Lord's, and the fulness thereof. ²⁷ If any of them that believe not bid you to a feast, and ye be disposed to go; whatsoever is set before you, eat, asking no question for conscience sake. ²⁸ But if any man say unto you, This is offered in sacrifice unto idols, eat not for his sake that shewed it, and for conscience sake: for the earth is the Lord's, and the fulness thereof:"
Background: The Christians in Corinth suspected the meat in the Gentile market, even clean ones, had been offered to idols. Therefore they avoided those completely (but they might have kept their own flocks). But Paul said it was alright as long as the they did not know that the offering to idol had been made.

Galatians 2: "(KJV) ¹² For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision. ¹³ And the other Jews dissembled likewise with him; insomuch that Barnabas also was carried away with their dissimulation. ¹⁴ But when I saw that they walked not uprightly according to the truth of the gospel, I said unto Peter before them all, If thou, being a Jew, livest after the manner of Gentiles, and not as do the Jews, why compellest thou the Gentiles to live as do the Jews?"
Not all gentiles ate unclean meat. Plus he must be doing something different that made the gentiles know he was a Christian although he ate and lived with them.

Colossians 2: "(KJV) ⁸ Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ."," ¹⁴ Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross; ¹⁵ And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it. ¹⁶ Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: ¹⁷ Which are a shadow of things to come; but the body is of Christ."
Background: The Christians in Colosse were trying to keep Jewish ceremonial law, for example offering sacrifice to atone sin, out of human tradition or 'just following the majority'. In verse 16, the whole context talks about ceremonies so it is illogical to regard 'meat and drink' here as clean or unclean. And the Bible does not talk about unclean drink anywhere. Colossians 2:16 is quite similar to Ezekiel 45:17 "(KJV) And it shall be the prince's part to give burnt offerings, and meat offerings, and drink offerings, in the feasts, and in the new moons, and in the sabbaths, in all solemnities of the house of Israel: he shall prepare the sin offering, and the meat offering, and the burnt offering, and the peace offerings, to make reconciliation for the house of Israel." So the meat and drink here are either about the offering or about the feasting in the festival.

1 Timothy 4: "(KJV) ³ Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth. ⁴ For every creature of God is good, and nothing to be refused, if it be received with thanksgiving: ⁵ For it is sanctified by the word of God and prayer."
Background: The place where Timothy was had people teaching not to marry and not to consume meats, including clean meats. Verse 4 simply states that everything God have created for consumption purpose is good. Verse 5 states that the clean meats are blessed by word of God (on the contrary those meats which are not created for consumption are damned by word of God the Bible) and can be further blessed by meal prayer. It does not mean unclean meat can be made clean by reading out the Bible and prayer.

Last word
If there is one creator of the universe, then He should be the best nutritionist. He receives no glory if we are sick (due to our own choice). Eating unclean meat is not a matter of sin or no sin but definitely is a matter of health or no health. Nevertheless, for sure He does not want us to starve to death if there is no food but just unclean meat. Also note that although meat of unclean animals can spread disease, the disease will not spread if humans do not eat them so we should not put hatred on those animals.

Exodus 15: "(KJV) ²⁶ And said, If thou wilt diligently hearken to the voice of the LORD thy God, and wilt do that which is right in his sight, and wilt give ear to his commandments, and keep all his statutes, I will put none of these diseases upon thee, which I have brought upon the Egyptians: for I am the LORD that healeth thee."

(Source of pictures found in this article: Wikipedia) (I do not represent any church in this article)
Download: Report by David Macht