Commentaire sur Le Lévitique 15:5

ואיש אשר יגע במשכבו AND WHOSOEVER TOUCHES HIS PLACE OF REPOSE [SHALL WASH HIS GARMENTS … AND BE UNCLEAN UNTIL THE EVENING] — This tells us regarding the object on which he lies down that it is more stringent than an object touched by him in any manner other than lying on it — for this (the former) becomes a primary source of uncleanness (אב הטומאה) communicating uncleanness to any man who touches it, he in turn making unclean the garments he happens to wear at that moment, while anything touched by him that is not his bed becomes only a secondary source of uncleanness of the first degree (ולד הטומאה) communicating uncleanness only to food and drink (but not to men or garments) (cf. Sifra, Metzora Parashat Zavim, Section 2 1).

Secondary source of uncleanness. I.e., the bed that the zov touches is a primary source of impurity, and it causes impurity to a person [who in turn] causes impurity to clothing that he is wearing, as it is written, “shall wash his garments and bathe in water.” However, something which is not specifically [used] for lying down and is touched by the zov, is not a primary source of impurity. Rather, it is a secondary cause of impurity, which is only a first. When in contact with the zov, the item causes impurity to a vessel which it touches that is purified by immersion; but it does not cause impurity to a person or earthenware vessels. After the zov separates from it, the item only causes impurity to food or drink. The meaning of “more severe than touch” is [that it is more severe than touch that is] not [associated] with lying down.

ואיש אשר יגע, “and a man who touches;” he contaminates his clothing at the time he touches his bed, his clothing will become contaminated. When he separates from the bed, seeing that his status of ritual contamination is relatively mild, i.e. ראשון לטומאה, one step removed from the impurity’s origin, his clothing had not become contaminated also.