Leviticus 15:23

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 ^F9 speaks of menstrues as very infectious, or worse, to various creatures and things, in a natural way.