Halakhah su Levitico 16:26

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)