Niddah 2: From When is a Woman a Niddah?

It is hard to believe that we are entering the last Massechet of Daf Yomi.  Massechet Niddah is devoted to the times when a woman is in a state of niddah, ritual impurity.  Men can also become ritually impure, but there is a greater focus on women's ritual impurity likely because this happens (usually) for at least a week of each month.  Further, ritual impurity of this sort is transmitted to objects that women touch or sit upon, and so the rabbis wanted to create guidelines and rituals that were very clear regarding niddah.

In our first Mishna, we learn about Shammai's view: women are ritually impure from the moment that they examine themselves with a cloth and find blood.  Hillel disagrees, saying that women are retroactively ritually impure - from the time that they last checked for blood until they checked again and found blood.  This might be several days.  The Rabbis say that women are retroactively ritually impure from only 24 hours before checking and finding blood.  If a woman checks herself twice each day, the rabbis then allowed women to be considered retroactively niddah for only 12 hours.

Shammai is usually the sage who is more stringent and Hillel is usually considered to be more lenient.  This is one of those times where the Sages do not follow their usual patterns.