Leviticus 15:4
Any bed the man with a discharge lies on will be unclean, and anything he sits on will be unclean. The contact-impurity from the man with a pathological discharge extends to everything he lies on or sits on: his bed and his chair or seat become sources of secondary impurity. The transmission of impurity through the surface that mediates between the person and others communicates the contagious character of this category of impurity. The bed and the seat that carry the impurity of the man with the discharge become impurity sources for everyone who subsequently touches them.