When we talk about the laws of ritual impurity as it related to nidda, there are two separate elements that must be discussed. On the one hand, a menstruating woman becomes ritually impure, a situation that forbids her from consuming kodashim, entering the Temple precincts and so forth. On the other hand, the menstrual blood itself is ritually impure and someone who comes into contact with it becomes tameh.
The connection between these two elements of ritual impurity is unclear. According to some, these two things are intimately related, so that menstrual blood is only ritually impure if the woman is rendered a nidda. In the event that the flow of menstrual blood does not render the woman a nidda, for example if the blood was not secreted directly from the vaginal area but was removed in some other way, then the blood would not be tameh. Others argue that these should be viewed separately, and even if the woman is not ritually impure, nevertheless the menstrual blood is tameh.
The seventh perek of Massekhet Nidda, which begins on today’s daf focuses on the status of the menstrual blood itself, dealing with such questions as whether the blood must be in a liquid state – parallel to the bodily secretions of a zav that are only ritually impure when they are wet – in order to be considered tameh.
According to the Mishna on today’s daf, menstrual blood is similar to the flesh of a corpse, and both of them will be considered ritually impure whether they are wet or dry. The Mishna makes no distinction with regard to the level of dryness, but in the continuation of the Gemara on tomorrow’s daf we find a disagreement on this matter. Resh Lakish rules that if human flesh dries out to the extent that it flakes apart, it will no longer be tameh; Rabbi Yohanan disagrees, ruling that under all circumstances it remains tameh. It is not clear whether or not the same disagreement applies to menstrual blood, as well.