These notes are taken from the New International Version Bible Commentary:
Leviticus 11:1 – 15:33
The Levitical laws of cleanness symbolized spiritual cleansing and set Israel apart from the surrounding nations. The spiritual and the hygienic reasons for the laws are remarkably valuable in the area of public health. In general they protected Israel from bad diet, dangerous vermin, and communicable diseases. Only recently have better laws of health been possible with the advance of medicine. These were rule-of-thumb laws that God gave in his wisdom to a people who could not know the reason for the provision.
First, the laws protected Israel’s diet. Some of the food forbidden was good some of the time, but not unless it was properly prepared. Pigs spread trichinosis; rabbits spread tularemia. The fish classified as clean are normally free-swimming, whereas scaleless and finless fish are usually bottom feeders and therefore susceptible to a great many more parasites. Cows, goats, and sheep are safe to eat under all ordinary circumstances and are economical to raise. The horse and camel were too uneconomical to use for meat.
The Hebrews were not only to avoid eating unclean animals; they were not to touch their dead carcasses. Thus the laws automatically helped control vermin. Common unclean animals would be spiders, flies, bugs, rats, and mice. A dead rat in a Hebrew house was not overlooked. It was carefully taken out and buried. In an effort to avoid such problems, the Hebrew housewife would normally keep a clean house.
The word “leprosy” used in older translations of chs. 13-14 is now generally recognized to include several “skin diseases” showing a rash, such as measles, smallpox, and scarlet fever. For any such disease there was a quarantine period with weekly examination until the patient was well. Likewise, the laws on sexual uncleanness protected Israel from venereal diseases and from childbed fever.