And if it [be] on [her] bed, or on anything whereon she sitteth,.... That is, if any person or thing should be upon her bed or seat; a vessel on her bed, or a vessel upon a vessel, as Aben Ezra expresses it:
when he toucheth it; that person or thing that should be on her bed or seat, as well as touch her bed or seat:
shall be unclean until the even; in a ceremonial sense; so defiling was a woman in such circumstances, and to whom the Scriptures often compare unclean persons and things: and Pliny i speaks of menstrues as very infectious, or worse, to various creatures and things, in a natural way.
i Nat. Hist. l. 7. c. 15.