“And you shall live by them” (Vayikra 18:5). The mitzvot of the Torah are meant to enhance life, adding meaning and sanctity to our sojourn on earth. The Torah is an eitz chaim, a tree of life, providing beautiful fruit year after year, generation after generation. Torah and death are incompatible. Thus kohanim, those tasked – at least in Temple times – with the teaching of Torah, were forbidden to come in contact with death. Death is defiling, with a dead body classified as the avi avot hatumah, the grandfather of tumah, rendering all who come in contact with a corpse tameh. The extent to which tumah can be transferred depends on how far removed one is from the original source of the tumah, and the level of holiness of the object under discussion.
One can contract tumah in one of three ways: maga, masa and ohel, by touching an object of tumah, by carrying it (even without actually touching it, i.e., in a knapsack) or by being in the same room as the object of tumah.
As tumah is transferred, its power diminishes. A person, or even food and utensils that come in contact with a human corpse become an av hatumah, father of impurity. Ashes from the red heifer must be sprinkled upon the person on the third and seventh day of one's contracting tumah and one must immerse in a mikvah to complete the purification process. An av hatumah can create a rishon letumah, a first-level tumah; a rishon creates a sheni, a second-level tumah; and on it goes, up to and including a chamishi letumah, a fifth degree of impurity.
However, only a parah adumah, red heifer, is susceptible to this fifth degree of tumah. A korban is susceptible to one degree less, such that it becomes disqualified even it is a fourth degree of tumah. Terumah, the approximately 2% agricultural tithe given to and to be eaten only by kohanim and their families (including non-Jewish slaves), is invalidated and hence, must be burned, even up to a third degree of tumah. Ma’aser sheni, the second tithe that the farmer is to eat in Jerusalem, is susceptible only as far as second-level tumah and non-consecrated foods, chulin, can become a rishon letumah and no more.
With tumah connected to death, it is not surprising that a women’s menstrual flow creates tumah, indicating as it does the loss of potential life. Just as there is nothing wrong with becoming tameh and it is often a mitzvah to do so, the fact that a menstrual flow creates tumah is perfectly normal and healthy.
The Torah distinguishes between two types, or shall we say two times, of menstrual flow. A woman who has a regular menstrual flow becomes a niddah, a status that lasts for seven days. At the end of the seven days the woman may go to the mikvah, provided her blood flow has stopped. Only one classified as a zava gedolah must wait “seven clean days” without bleeding before going to mikvah. One becomes a zava gedolah when there is bleeding “not during her time of niddah” (Vayikra 15:25) over a period of three consecutive days. If there is bleeding for one or two days, then one can go the mikvah after just one clean day. In addition to going to the mikvah, a zava gedolah must bring “two doves or two pigeons…one as a chatat, a sin offering, and one as an olah, a burnt offering” (Vayikra 15:29,30).
Determining when bleeding is part of the natural cycle, thus “only” making one a niddah, or not part of the natural cycle, making one a zava, can be most complicated and does not necessarily correspond to the natural menstrual cycle of most women. In order to avoid any mix-ups, “the daughters of Israel were strict with themselves that even if they see blood the size of a mustard seed, they sit [wait] for seven days” before going to mikvah (Niddah 66a), thus effectively turning a zava ketana to a zava gedolah and eliminating any distinction between a niddah and a zava gedolah.
One of the fascinating aspects of this practice is that it came from the bottom up. It was the women themselves, not the rabbis, who worried about any possible confusion. To say that this is one of the most impactful decisions in the history of Jewish law is no understatement, greatly increasing the period of time husband and wife must not be intimate.
The laws of ziva applied equally to males who had unnatural seminal emissions. Here, too, if there was an emission for one or two days, one could go to mikvah after just one clean day, but if the the bodily fluids continued for three days (or if one had three emissions on the same day), one could go to mikvah only after seven clean days. And here, too, there was a rabbinic addition – this one instituted from the top down by Ezra HaSofer as he established the contours of Jewish life at the beginning of the second Temple period – that one who had any seminal emission, i.e., after relations with his wife, could not study Torah nor engage in tefillah until after going to the mikvah (Brachot 22a). Tellingly, this decree backfired, leading to less prayer and Torah study, and was soon repealed.
If tumah is contact with death, tahara must be the celebration of life. And that is why it takes a mikvah to make one pure. Water is the prerequisite for any form of life to exist. It is thus not surprising that Torah is compared to water, without which the Jewish people cannot exist. And hence, Ezra decreed that Jewish communities not go three days without the public reading of the Torah (Bava Kamma 82a), a decree that has been faithfully observed for the past 2,500 years.
 While there was no halachic reason to be concerned that chulin meat may become tameh, there were many who were careful to eat chulin betahara, regular meat in a state of purity, either as a precaution lest they touch other foods where tumah would be an issue or, more commonly, because despite no legal need to do so, they wanted to remain in a state of tahara.
 This is an example of a sin offering brought when no sin has been committed, much like the case of a new mother, who must do the same. We will have to leave for another time a discussion as to why this may be so.
 Presumably, when the Temple is rebuilt and korbanot are once again brought (that, too, is a subject for another time), we will distinguish between a niddah and a zava.