Halakhah su Levitico 15:13

That is that He commanded us that any man who is healed from a discharge offer a sacrifice. And this is the sacrifice of the zav; and he is lacking [full] atonement until he offers it. And that is His, may He be exalted and may His name be blessed, saying, "When one with a discharge becomes clean of his discharge [...]. On the eighth day he shall take two turtledoves" (Leviticus 15:13-14). (See Parashat Metzora; Mishneh Torah, Offerings for Those with Incomplete Atonement 1-3.)

That is that He commanded us that we immerse in the mikveh waters, and that we then become pure from one of the various types of impurities with which we have become impure. And that is His, may He be blessed, saying, "and he shall bathe in water" (Leviticus 15:16). And it comes in the tradition that it is water that all of one's body can enter, and that is the measurement of a mikveh - unless the waters are running waters, which have no measurement; as is explained in the analysis of this commandment. And it is among the stipulations specifically for the zav, that the waters that purify him be running - as in the Torah, it says, "living waters" (Leviticus 15:13). And the intention of the statement that immersion is a positive commandment is not that anyone impure is obligated to perforce be purified, in the same way that anyone covered by a cloak must make tzitzit, or anyone with a home must make a parapet. Rather what is intended is the law of immersion - and that is that the Torah told us that anyone who wants to be purified from his impurity will only complete it with immersion in water; and he will then be purified. And the language of the [Sifra] (Sifra, Acharei Mot, Section 5:3) is, "'And he shall bathe [...] in water' (Leviticus 16:26) - I might think that it is a decree of the King (that he must immerse). [Hence] we learn to say, 'and then he may come to the camp' - from his impurity." This hints to the principle that we have explained - that the law is only that anyone who wants to become pure do this. That is the law, and that is the commandment - and not that he be obligated perforce to immerse. Rather, anyone who wants to remain impure and not enter the camp of the Divine Presence for a certain time is allowed [to do so]. And it has already appeared in the books of truth that one who becomes impure and immerses is purified, but he does not complete his purification until the sun goes down for him. And it also appears in the accepted tradition that when he immerses, his naked flesh must be in contact with the water itself and that there not be anything separating between him and the water. Behold this commandment has been explained to you - and that is the commandment of immersion; and it includes the regulations of the mikveh, the regulations of bathing and the regulations of one who has immersed that day (before sunset). And this law is explained in Tractate Mikvaot and in Tractate Tevul Yom. (See Parashat Metzora Mishneh Torah, Immersion Pools 1-11)

And if you shall ask, why did they, may their memory be blessed, obligate to count the seven [cycles of] seven from that which is written, "And you shall count for yourself"; and [yet] we have never seen that the zav counts the days of his counting, nor the zavah the days of her counting, and even though it is written about them, "and he shall count for himself" (Leviticus 15:13), "and she shall count for herself" (Leviticus 15:28) - besides that they are obligated to pay attention to the days, but not that they be obligated to count them orally and recite a blessing on their count - the answer to this thing is what I prefaced at the beginning of my book: that every matter of the Torah is dependent upon the traditionally received explanation. And for the one that does not know this, how many verses will appear to be the opposite of one another, and how many difficulties and contradictions will arise? But for the one that knows it clearly, he will see that all of 'its ways are the ways of pleasantness and all of its paths are peace' and truth. And so, the tradition came to us that the command of, "And you shall count for yourself," of Jubilee requires an oral counting; whereas the command of counting, written about the zav and the zavah, is only paying heed to the days - and such is the practice of all of Israel in every place. "And even though they are not prophets, they are the children of prophets." And similar to this matter is that which we found in the Torah about the expression, remembering. As remembering is written about Amalek, and remembering [is written] about the [incident] of Miriam, and remembering is also written about the matter of leaving Egypt: And the tradition came to us about the remembering of Egypt to do it orally - and as they, may their memory be blessed, said (Berakhot 21a), about the blessing of "True and firm" (which mentions the leaving of Egypt), "It is from Torah writ." But with the other rememberings, it is enough for us with just remembering of the heart and paying heed to the things.