Reference for Bekhorot 75:23

אי בטומאה דאורייתא ה"נ הכא במאי עסקינן בטומאת משקין דרבנן

for according to the biblical law, food cannot make a vessel unclean nor can unclean liquid make a vessel unclean, and only the Rabbis have declared uncleanness on account of the liquid of a zab and a zabah.<span class="x" onmousemove="('comment',' One afflicted with gonorrhoea. His or her spittle is one of the direct causes of levitical impurity and it makes a vessel unclean biblically, whereas other unclean liquids cannot do so, but only make the vessel rabbinically unclean.');">¹⁵</span>

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