Jamieson, Fausset and Brown on Leviticus 11:31-35

31-35. whosoever doth touch them, when ... dead, shall be unclean until the even--These regulations must have often caused annoyance by suddenly requiring the exclusion of people from society, as well as the ordinances of religion. Nevertheless they were extremely useful and salutary, especially as enforcing attention to cleanliness. This is a matter of essential importance in the East, where venomous reptiles often creep into houses and are found lurking in boxes, vessels, or holes in the wall; and the carcass of one of them, or a dead mouse, mole, lizard, or other unclean animal, might be inadvertently touched by the hand, or fall on clothes, skin bottles, or any article of common domestic use. By connecting, therefore, the touch of such creatures with ceremonial defilement, which required immediately to be removed, an effectual means was taken to prevent the bad effects of venom and all unclean or noxious matter.