פירוש על זבים 5:2

כל הנישא על גבי הזב – the uppermost part of the person with a flux/gonorrhea and even if the person with a flux/gonorrhea did not touch him and even if there are many things that interpose between him and between the person with a flux/gonorrhea, everything is ritually impure, as it is written (Leviticus 15:10): “Whoever touches anything that was under him [shall be impure until evening].” What is underneath him? If you were to say, underneath the person with flux/gonorrhea, that is his lying, but rather with anything that the Zav/person with flux will be underneath him, that is, the uppermost part of the Zav. And this applies to all things, even with something that is not normally upon him, and not with that is similar to what is below him, for it is not practiced other than something that appears to him.

Introduction Today's mishnah provides some general rules as to how a zav defiles things that are either above him or below him that he does not touch.

חוץ מהן הראוי למשכב ולמושב – and the same law applies for what he is riding upon, but that which is not appropriate is pure, as for example, if he inverted a Seah which is not made for sitting and he sat upon it, it is possible that it would be impure for sitting, as the inference teaches us (Leviticus 15:6): “Whoever sits on an object on which the one with the discharge has sat,” that which is special fro sitting, excluding this that we say to him: “stand and perform our work.”

They said yet another general principle: All that is carried above a zav becomes defiled, but all those things above which he is carried are clean except objects on which he can sit or lie upon, and a person. This section describes the general rule. Anything above a zav is defiled but things that are below him are defiled only if they were meant for sitting or lying upon, except for a person who carries a zav who is also defiled. The rest of the mishnah will illustrate this principle.

נדבך (a frame carried to the building ground with the tools and vessels above and under it) – a row of the building of the wall, like (Ezra 6:4): “With a course of unused timber [for each three courses of hewn stone].”

How so? If a zav had his finger beneath a layer of stones and one that was clean was above, he conveys uncleanness at two [degrees of remove] and disqualifies [terumah] at one [further remove]. If he separated [from the source of uncleanness] he still defiles at one remove and disqualifies [terumah] at yet one [further remove]. The mishnah now begins to demonstrate how this defilement is conveyed. A zav has his finger underneath a row of stones (ouch!) and on top of the row of stones is a pure person. The pure person is considered to be carried by the zav. The pure person now becomes a "father of impurity." Anything he touches will have first degree impurity and anything touched by it will have second degree impurity. If the object with second degree impurity touches terumah the terumah is disqualified (see Kelim 19:4). This is in general a description of the defiling power of one who is considered to be a father of impurity. If the pure person moves away from the row of stones or the zav removes his hand then the pure person will only have first degree impurity. Anything he touches will have second degree impurity and any terumah that it touches will be disqualified.

הטהור מלמעלה – on the frame/course of stones and weighing down upon the finger of the person with flux/gonorrhea (i.e., the Zav), for now it is carried above the Zav and is considered a primary source of ritual defilement.

If the unclean one was above, and the clean person below, he defiles at two [removes], and disqualifies [terumah] at yet one [further remove]. If he separated [from the source of uncleanness], he defiles at one [remove] and disqualifies [terumah] at yet one [more remove]. The same rules apply if the zav was on top of the row of stones and the finger of the pure person was below.

מטמא שנים ופוסל אחד – it defiles first degree [of ritual impurity] and second degree [of ritual impurity], and renders heave-offering/priest’s due] impure with one further remove.

If foods or liquids, or objects on which he could sit or lie upon or other articles were above, they defile at one remove, and disqualify [terumah] at one [further remove]. If they had become separated [from the source of uncleanness], they defile at one [remove] and disqualify [terumah] at one [more remove]. If the zav is below and something that is meant to be sat upon or food or any article (called in Hebrew a madaf) was above, he defiles them with only first degree impurity. This doesn't change if the source of impurity is removed. [I should note that I have explained this according to most commentators, but they printed editions of the Mishnah have a different reading].

פירש – the pure person came down from the course of stones/frame, or that the person with flux/gonorrhea (i.e., the Zav) removed his finger from underneath the course of stones/frame. It defiles the pure with one remove and invalidates one remove, because the pure thing is not other first degree of ritual impurity for since it separates, it further does not defile clothing and makes heave offering/priest’s due second and third remove.

All objects fit to sit or lie upon that were below defile at two [removes], and disqualify [terumah] at one [more remove]. If they had become separate [from the source of uncleanness], they defile at two [removes] and disqualify [terumah] at one [more remove] Foods and liquids and other articles that are below, remain clean. However, if the zav is above and something meant to be sat or lied upon is below him, he defiles them such that they become fathers of impurity. Even if the source of impurity is removed, they remain fathers of impurity. However, he only conveys impurity to things that are meant to be sat or lied upon. If below him are foods, liquids or other articles not meant to be sat upon, they remain pure.

הטמא מלמעלה – on the course of stones/frame, and the finger of the pure person is underneath the course of stones/frame and the impure person weighs down upon him, for the Zav/the person with a flux/gonorrhea is carried upon it, therefore it defiles two removes and renders unfit one further remove.

והמדף – all the vessels that are not lying or sitting are called מדף/indirect contact for conferring ritual impurity.

מלמעלה – [above] the course of stones/frame, but the finger of the Zav/person with flux/gonorrhea is from below, they are not other than first degree of ritual impurity,

and they defile one remove and renders unfit [still] one [further] remove, but after they have separated, also this is the law.

פירשו מטמאין שנים ופוסלין אחד – the person who lies and who sits is always a primary source of ritual defilement, even though they (i.e., the defiling agents) have separated themselves.

טהורים – as we have stated that all that the Zav/person with flux/gonorrhea is carried upon is pure, except for from lying and sitting and the human being.