Halakhah su Kelim 1:1

The laws of the commandment: For example, that which they, may their memory be blessed, said (Mishnah Kelim 1:1) that eight swarming creatures are called a primary source (av) of impurity, meaning to say that they render a man and vessels impure by touch; and if the vessels are of clay, they render them impure once they enter its space (hollow). And this is the distinction that there is in the matter between that which is called a primary source and that which is called a derivative (velad) - as that which is called a primary source renders a man and vessels impure, but that which is called a derivative does not render a man and vessels impure. And the swarming creatures only render impure after they die, as it is stated (Leviticus 11:31), "in their being dead, he shall become impure." And it is specifically these swarming creatures that render impure, but all other swarming creatures - the snake, the scorpion and all that are similar to them - do not render impure at all. And the law of the swarming creatures is that they do not render impure by carrying without touching [directly]; and that one that touches them does not render his clothes impure when they are upon him at the time that he touches them. And the measure of the impurity of the swarming creature is like [the size of] a lentil. As behold, we found that the Torah renders them pure undifferentiatedly, and there are some of these swarming creatures that are only like a lentil. And all of the swarming creatures combine to [the measure of] like a lentil - meaning to say, even a little of this one and a little of that one combine to [form the size of] a lentil, to render impure; such that you not say that the liability is only like a lentil from one [creature] by itself (Meilah 15b).

And afterwards, the priest shaves the metsora. And how does he shave him? He passes the razor over his visible flesh - and even [on] the underarm and the pubic area and all the rest of the body - until he becomes [smooth] like a gourd, as it is stated (Leviticus 14:9), "all of his hair." If so, why does it state, "his head, his beard, and his eyebrows?" To include everything that is like them and to exclude hair that is in the nose, since it is not seen. And afterwards he washes his clothes and immerses, and is purified with this from passing on impurity from his resting and sitting [upon something]; and he can come inside the [city] wall. And he counts seven days; and during those seven days, he is prohibited in sexual relations, as it is stated, "outside of his tent" - teaching he is forbidden in sexual relations. But a metsora'at is permitted in sexual relations. All of these seven days, he is still a source of impurity and renders a person and vessels impure with touch, but he does render impure with carrying - as behold it states (Leviticus 14:9), "And it shall be on the seventh day, etc. and he shall wash his clothes" - teaching that he was rendering clothes impure. And [just] like he was rendering clothes impure, so [too] was he rendering a man impure with touch - since all that renders a person impure, renders clothes impure; and all that does not render a person impure, does not render clothes impure (Mishnah Kelim 1:1). And on the seventh day, the priest shaves him a second [time] like the first shaving. And [the metsora] washes his clothes and immerses and is purified from rendering others impure. And behold he is like all who have immersed during the day, and can eat from the tithe; when his sun sets, he can eat from the priestly tithe; [and] when he brings his atonement, he can eat from consecrated foods. The slaughter of the bird, the shaving and the sprinkling are during the day, but all the other procedures are whether during the day or during the night. These [three things] are with men, but all the rest are even with women. These are with priests, but all the rest are even through an Israelite. And it is a commandment for the same priest who rendered him impure [to render him pure, as it is stated (Leviticus 13:59)], "to render him pure or to render him impure." And the rest of its details are in Tractate Negaim.