According to Biblical law, when a woman experiences her menstrual cycle she is a nidda whether she bleeds only once or many times over a period of seven days. At the end of seven days she immerses in a mikva and is rendered ritually pure. A zava is a woman who experiences a flow of menstrual-type blood during a time of the month when she is not due to experience menstrual bleeding. The eleven days that follow are called yemei ziva; If she experiences vaginal bleeding during that time she is rendered a zava.
The laws of zava differ from those of a nidda. If a zava experiences bleeding just once or twice during that period, she is deemed a zava ketana who will “keep watch a day for a day” – she must check that she is free of bleeding one day for each day that she bled. After experiencing bleeding on a third day, however, the woman is considered a zava gedola and is obligated to wait a full seven days without bleeding. At that time she can immerse in a mikva and she will be permitted to her husband. The next day she must bring a sacrifice as part of her purification process, which will allow her to enter the Temple and consume sacrifices (see 15:25-29).
On today’s daf we find a statement made by Rabbi Zeira that has led to a long-term stringency regarding the laws of nidda. Rabbi Zeira related that:
Jewish women are stringent with themselves that even if they observe a drop of blood of the size of a mustard seed they wait on account of it seven clean days.
This stringency effectively forces every woman to keep the rules of a zava who must wait seven clean days, even when she is almost certainly a nidda who should be able to immerse seven total days after her menstruation began (even if the bleeding continued throughout the week). This tradition has taken on the power of established law and Rambam, Shulḥan Arukh and others describe it as obligatory, even as they recognize that it is a severe stringency.