There are eight crawling animals (“sheratzim”) listed in verses 29 and 30. As with the list of non-kosher birds, exact identification is difficult. Suggestions for some of these animals – with Hebrew names like anakah, l’ta’ah and tinshemes – include chameleons, ferrets, hedgehogs, weasels and moles. Whatever these animals are, the list definitely includes a mixture of mammals, reptiles and amphibians. (Rashi identifies the tzav as frog-like, presumably meaning a toad.) There may even be mollusks on the list, since Rashi identifies chomet as a snail or a slug. Special laws beyond kashrus apply to these eight animals.
The eight sheratzim are ritually unclean and capable of transmitting this impurity. The corpses of these animals are called an “av hatumah” (primary source of ritual impurity) and can contaminate both people and objects. In a special case, earthenware vessels are only contaminated through their interior space. The transmitted contamination results in what’s called a “vlad hatumah” (the offspring of impurity); the person or thing contaminated in this fashion does not “pass it on” any further.
Sheratzim only impart impurity through contact (or through the interior cavity of a clay vessel). If a person carries one without touching it, he remains clean. Furthermore, only the person who touches them becomes unclean; this uncleanliness does not extend to his clothes. It does not have to be a complete carcass to impart this impurity; a lentil-sized piece of one of these animals (or made up of several of these animals cumulatively) is sufficient to do the trick.
The person who becomes impure through one of the eight sheratzim must wait until nightfall and immerse in a mikvah. The same holds true for utensils, but earthenware cannot be purified and must be disposed of.
These laws only apply to the eight animals named by the Torah. Other animals, such as scorpions, do not impart such impurity no matter how creepy crawly they are.
The laws of ritual impurity are among the most enigmatic and elusive in the Torah. The best we can figure is that, like non-kosher food, these things are spiritually harmful in some fashion. The Talmud in Yoma (39a) engages in exposition on Leviticus 11:43. There, it says “v’lo sitamu bahem v’nitmeisem bam” – “do not make yourselves impure through them so that they defile you.” The school of Rabbi Yishmael homiletically interprets this verse as if it says, “…v’nitamtem” – “so they do not make you confused” (or stupid or closed-hearted). Clearly, the Sages equated ritually impurity with negative traits. To avoid the former is to avoid the latter.
This mitzvah applies to men and women in all times and places. (Though we are currently incapable of practicing most laws of ritual purity, the corpses of these animals still render us ritually impure today.) The laws of the sheratzim are discussed in the Talmud in tractate Chulin (126b-128b) and in the Mishna in tractate Keilim (see 1:1, et al.). This mitzvah is codified in the Mishneh Torah in the fourth chapter of Hilchos Shar Avos HaTumah. It is #97 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.
While ill-advised, it is not sinful to become impure, even intentionally. Accordingly, the Ramban (Nachmanides) does not consider this one of the 613 mitzvos. Rather, he feels that the Torah has instructed us what we may not do when in a state of ritual impurity. Here, Nachmanides feels, the Torah is just defining for us what that state is.