The 108th mitzvah is that we are commanded to follow the laws regarding mei niddah: that in some cases it will make [something which was previously tameh] tahor, and in other cases make [something which was previously tahor] tameh, as explained in the section dealing with this mitzvah.
You should be aware that the 13 forms of tumah listed above — namely, neveilah, sheratzim, foods, niddah, a woman who has given birth, a leprous person, a leprous garment, a leprous house, zav, zavah, semen, a dead body, and mei niddah — as well as the purification procedure for each, are all explicitly stated in Scripture. Each mitzvah contains numerous verses, laws and conditions, as written in the Torah portions Vay'hi BaYom HaShemini, Ishah Ki Tazriah, Zos Tihiyeh, and Vayik'chu Eilechah Parah Adumah. These four Torah portions contain all the verses which speak of the forms of tumah.
However, the laws governing these forms of tumah in general, and the details regarding each one in particular are all found in Seder Taharos. Some categories are covered in particular tractates, such as tractate Taharos, Machshirim and Uktzin. These three tractates deal solely with, and were written to explain the tumah of foods. Any mention of other forms of tumah is incidental. Tractate Niddah includes the laws of tumas niddah, zavah, and a woman who has given birth. Tractate Kerisus also contains some laws of a woman who has given birth. Tractate Negaim contains the laws of leprous people, garments and houses. Tractate Zavim contains the laws regarding a zav, zavah and semen. Tractate Ohalos contains the laws of tumas meis. Tractate Parah contains the laws of mei niddah — when it conveys tumah and when taharah.
The tumah of neveilah and sheretz, however, are not limited to any particular tractates. Their laws are scattered throughout this Seder [i.e., Taharos], mostly and primarily in Keilim and Taharos. Tractate Ediyos also deals with many questions of this sort.
We have already written a commentary on this entire Sefer, i.e., Taharos, that makes it unnecessary to consult any other book for anything related to tumah and taharah.
