First, a little background on the Mosaic Law. The infant nation of Israel had just been freed from Egypt when God gave the law to Moses. At this time, the Israelites had been hip-deep in pagan religion to the point where it would have been impossible for them to keep God's regulations and become His chosen people that would produce the messiah. This is a nomadic society. There are no ovens, running water, or any sort of soap or disinfectant.
This is why the Bible has such strict guidelines on the topic of sanitation. The common practices of the day included using fecal matter to treat open wounds such as in Egypt, not burying in the sand. It wasn't until the 1900s that doctors began washing their hands between patients, thus reducing premature death rates. Laws on sanitation were not just limited to washing and excrement disposal. (Deuteronomy 23:12, 13) It included the quarantine of infected individuals (Leviticus 13:4-8 21, 26) and the dangers of blood (Genesis 9:4; Leviticus 7:26; 17:12; Deuteronomy 12:23-25).
The most commonly known "unclean foods" today include pork and shellfish (Leviticus 11:23, 3, 10). In a nomadic society, the technology required to make the consumption of pork safe did not exist. Campfires could not produce adequate heat to kill parasites, bacteria, and other pathogens. They lacked ovens, which could raise the internal temperature of the meat to 160*F (71*C), which the Food Safety and Inspection Service (FSIS) recommends. Undercooked or untreated pork may harbor pathogens or it can be easily recontaminated after cooking. It is known to carry some life-threatening diseases such as pork tapeworm and trichinosis. The pig is a carrier of various parasites, such a roundworms, pinworms, hookworms, etc. One of the most dangerous and common is Taenia solium, a type of tapeworm. Left untreated, it is fatal. In developed countries where we have freezers and ovens that can cook up to 500*F, the risk of infection from pork is extremely low. In the absence of more advanced technology, the best protection from these diseases was simply not to eat it.
Shellfish is a general term that includes any aquatic invertebrate with an exoskeleton, including shrimp, lobster, and crab. It also includes fish closely related to insects, such as squid, octopus, oysters, and clams. Anyone who has ever eaten at a restaurant has seen the disclaimer that says the restaurant is not responsible if the guest requests undercooked meats or seafood. Even sushi from high caliber restaurants can cause parasitic infections such as hookworm.
In addition to the danger of parasites and bacteria, shellfish are a common high allergen food. During this time period, there were no medications to combat a severe allergic reaction that could lead to death. Ask anyone with a severe peanut allergy how scary it was to find out for the first time that a life-threatening reaction is possible. He would want to be spared that, wouldn't he? Wouldn't a loving God want to spare his people from that? Yes, he would, just like any loving parent would carefully create safe meals for her family.
The End of the Mosiac Law or "Why Pork is Okay Now"
The Mosaic Law ended with Jesus' death. (Matthew 26:26-28; Hebrews 7:12; ) Technology had vastly improved, making the consumption of these foods safe. Animal sacrifices were no longer required because they had led up to Jesus' death. (Romans 5:12-15; Hebrews 10:1-4) There were only a few restrictions: blood, fornication, and idolatry. (Acts 15:19, 20) Otherwise, Christians are now governed by conscience, rather than law. (Romans 2:14-15) We can study the Mosaic Law and use that as a guideline to direct our consciences, but we are not required to hold it to the letter because an individual who does so may reason that, even though they are physically capable of doing more, he can say "I've done what the law says. I don't have to do more."
Everything in the Mosiac Law had a purpose. The various laws either served as a protection, be it sanitary or preventing the influence of pagan nations; led up to the Christ; were directly related to religious practices regarding worship that God approved; visually set them apart from nearby powerful nations; or showed how to treat each other. Despite the function of the Mosaic Law being obsolete after the death of Christ, the Bible is not outdated. It is a wonderful, harmonious book that is without parallel. The Bible's commands are not burdensome, but attainable, practical, and beneficial for all mankind. (1 John 5:3)