(Chs. 11-15) [of Leviticus] form an important section on ritual purity and pollution. An explanation now almost universally rejected is that the various laws in this section have hygiene as their basis. Although some of the laws of ritual purity roughly correspond to modern ideas of physical cleanliness, many of them have little to do with hygiene. For example, there is no evidence that the "unclean" animals are intrinsically bad to eat or to be avoided in a Mediterranean climate, as is sometimes asserted.
The Oxford Bible Commentary