The 109th mitzvah is that we are commanded to immerse in the waters of a mikvah and thereby be purified from whichever form of tumah previously existed.
The source of this commandment is G‑d's statement (exalted be He), "He must immerse his entire body in water." The Oral Tradition explains that [the phrase "his entire body" also teaches that] there must be enough water to cover his entire body. This is the minimum size of a mikvah unless the water is from a spring, in which case there is no minimum amount, as explained in the laws which deal with this mitzvah.
Among the conditions [governing the mikvah] is that only in the case of a zav is spring water required, as the verse states, "mayim chaim."
When we say that immersion is a mitzvah, this does not mean that any person who is tameh is required to immerse himself — as one who wears a four-cornered garment must put tzitzis on it, or that anyone with a house must build a fence around the roof. [When I say, "it is a mitzvah,"] I refer to the laws of immersion — that we are commanded that anyone who wants purification from tumah can do so only through immersion in water, after which he becomes tahor.
The Sifra says, "One might think that the phrase 'He must immerse in water' is a Divine decree [and that it is an absolute requirement to immerse]. The verse therefore says, 'then he can return to the camp' [after being purified] from tumah." This hints to the principle I just explained, i.e., that the mitzvah is just the law, i.e., that one who wants to be purified should take certain steps. This law is itself the mitzvah. This does not mean however, that there is an independent requirement to immerse — should he want to remain tameh and not enter the machaneh Shechinah for any period of time, he may do so.
The Book of Truth (i.e., the Torah) explains that even though after the person immerses he becomes tahor, his purification is incomplete until sunset. The Oral Tradition also explains that during immersion he must be naked and that his entire body must come in contact with the water. As our Sages put it, "The phrase, 'his entire body' teaches that there can be nothing intervening between his body and the water."
We have therefore explained that this mitzvah of immersion includes the laws of mikvah, of intervening substances, and t'vul yom. This mitzvah is explained in tractates Mikva'os and T'vul Yom.
