Before microscopic organisms or germs were discovered, God gave the ancient Israelites dietary laws which not only had spiritual significance, but would also protect them and help them to survive as a nation. These laws preserved the sanctity of Israel as God's holy people. Though the idea of uncleanness typified sin and defilement, it also protected the Israelites from diseases they may have otherwise become the victims of. Remember, all of this was given by God to Moses and Aaron thousands of years before infectious diseases were discovered, or the microscope was invented.
Following are a few examples:
"The rabbit, though it chews the cud, does not have a split hoof; it is unclean for you." (Lev. 11:6)
Tularemia (also known as rabbit fever or deerfly fever) is an infectious disease in ticks and rabbits that is caused by a bacterium (Francisella tularensis). The disease was first described in Japan in 1837. Approximately 150-300 tularemia cases are reported in the United States annually, with a majority of those from Alaska, Arkansas, Illinois, Oklahoma, Missouri, Tennessee, Texas, Utah, and Virginia. The bacteria F. tularensis is a hazard to laboratory staff that work closely with rabbits. Note: as few as 5-10 bacteria can result in disease.
"And the pig, though it has a split hoof completely divided, does not chew the cud; it is unclean for you. You must not eat their meat or touch their carcasses; they are unclean for you." (Lev. 11:7-8)
In ages past, pork could cause trichinosis. Modern food preparation techniques help prevent this. Pigs are also host to various parasites. In addition, pork is unhealthy since its even higher in fat than beef. For an explanation of trichinosis, see: http://en.wikipedia.org/wiki/Trichinosis
"But all creatures in the seas or streams that do not have fins and scales—whether among all the swarming things or among all the other living creatures in the water—you are to detest." (Lev. 11:10)
Unclean 'fish' such as eels generally contain more parasites than fish with scales. Even today, shellfish often causes food poisoning and enteritis. For example, see: http://www.ifremer.fr/docelec/doc/1992/acte-1595.pdf
"These are the birds you are to detest and not eat because they are detestable: the eagle, the vulture, the black vulture, the red kite, any kind of black kite, any kind of raven, the horned owl, the screech owl, the gull, any kind of hawk, the little owl, the cormorant, the great owl, the white owl, the desert owl, the osprey..." (Lev. 11:13)
Carnivorous animals readily transmitted infection in a warm climate where flesh decayed rapidly. Vermin and predatory birds were likely disease carriers.
Those kosher laws from God were like an ancient version of our Food and Drug Administration, protecting public health by preventing the consumption of unhealthy foods. Yet, their purpose was also to maintain spiritual purity, and served as another way to separate Israel, God's chosen people, from the idolatrous and pagan nations around them.