Joe Reichman asks:
Why didn't the Mishnhah mention Zav and Zava she is a Shomeres Yom k'Neged Yom he can become a Zav Gadol in the same day she needs Mikveh to become Tahor he can can use Mayim Chayim?
Joe Reichman, Silver Spring MD USA
The Kollel replies:
Tosfos (23b DH Mah) says that the Mishnah could also have said that a Zav can become a Zav Gadol on one day. Tosfos also lists other Halachos that the Mishnah could have mentioned. I presume that Tosfos means to say that the Tanna was Tanna v'Shi'er, in other words that the Mishnah does not want to say that this is a complete list rather that this is a partial list. The Gemara uses this answer in many places. This would also answer the other part of your question.
Dov Freedman