According to Chaza”l, there are seven liquids that can act as conduits for tumah. In order for a plant or food to contract tumah, it must first be touched by one of these liquids. Chaza”l learn out from the passuk, “And the blood of corpses they will drink,” in Parshas Balak, that blood is considered a drinkable liquid, thereby including blood as one of these seven substances.
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Rabbi Aharon Dovid Singer
Rosh Kollel, NMB Kollel
Director, South Florida Torah Initiative