Niddah 7:1-2

Niddah 7:1

Niddah blood and corpse flesh convey impurity both wet and dry; a zav’s discharge, his phlegm, his saliva, vermin, a carcass and semen all convey impurity when wet but not when dry. If soaking can restore these things to their previous condition, then they convey impurity both wet and dry. Soaking means in warm water for 24 hours. Rabbi Yosi says that if corpse flesh is dry and soaking cannot restore it to its previous condition, it is ritually clean.

Niddah 7:2

A vermin found in an alley conveys impurity retroactive to the time when one can say that he searched the alley and the vermin wasn’t in it, or to the time the alley was last swept. Similarly, a bloodstain found on a garment conveys impurity retroactive to the time when one can say that they last checked the garment and the stain wasn’t there, or to the time it was last washed. These things convey impurity both wet and dry. Rabbi Shimon says a dry vermin conveys impurity retroactively but a wet one only conveys impurity as far back as it could have been wet.