Commento su Parah 8:5

כל ולד הטומאה – as for example first-degree and second-degree, do not defile vessels, but the primary source(s) of ritual impurity. But liquid that was defiled in first-degree or second-degree defiles vessels, as a decree because of the liquid of a male with gonorrhea and/or a woman with a flux, such as his spittle, and his flux and his urine which are primary sources of ritual defilement.

Any derived uncleanness does not defile vessels, but [it does defile] a liquid. A derived uncleanness is one that is of either first or second degree uncleanness, meaning it had contact with something that was either a father of uncleanness or had first degree uncleanness. This level of uncleanness defiles liquids but not vessels.

If a liquid became unclean it defiles them. The liquid will then defile the vessel, because liquids convey uncleanness to vessels.

Behold [the vessel] would say [to the liquid], "That which defiled you did not defile me, but you did defile me. This again creates the paradox. The derived uncleanness was not strong enough to defile the vessel, but that which it defiled (the liquid) was strong enough to do so. The vessel is not happy with that liquid, and it lets it know!