Commentaire sur Le Lévitique 15:4

כל המשכב EVERY PLACE OF REPOSE — This means: everything that is fitted for sleeping on (thus excluding a door or plank etc.). One might, however, think that it includes anything intended for some other action also, as long as it can be used for sleeping and he actually slept upon it! It, however, states אשר ישכב — it does not state אשר שכב, “upon which one has slept" but אשר ישכב, “upon which one sleeps usually" — i.e. that which is intended to be used for this always, thus excluding that of which people can say to him, “Get up and let us to our work [with that upon which you are lying]" (Sifra, Metzora Parashat Zavim, Chapter 1 1-3).

אשר ישכב עליו, even if the (healthy) person lies under a stone not touching the bedstead of the afflicted person at all. The zav’s ritual impurity is still conferred upon him. The common denominator of the wide-ranging ability to confer impurity of zav, zavah, and niddah, the menstruating woman, are that they confer their own severe degree of ritual impurity to anything which lies underneath them even though not in physical contact with them. The only exception to this rule is if said afflicted party sits or lies on something that does not normally serve as a place to sit on or to lie on.

The verse says: “Upon which he lies.” Meaning: For instance, a bakers’ plank or large kneading trough.

אשר ישב [EVERYTHING] WHEREON HE SITTETH — It is not said here ‎יָשַׁב ‎‏ אשר, "on which he has sat", but אשר יֵשֵׁב, “on which one sits usually" — it speaks therefore of something that is intended to be used for this always (Sifra, Metzora Parashat Zavim, Chapter 1 3; Shabbat 59a).

Get up so that we can do. I.e., when it is being used for other work it is impossible that he could continue lying on it.