Why does the Torah write "mi'Besaro" (rather than 'mimenu')?
Rashi: Refer to 15:2:2:1*.
From whereabouts on the body must a man have an emission in order to be labeled a Zav?
Rashi: From the same location where he is a 'Ba'al Keri - the Amah (the Eiver Tashmish). 1
From the 'Mem' in "mi'Besaro" we learn that it is only from one spot on the body (and not from wherever it emerges), and from a Zavah we learn that, just as a woman is Metamei be'Zivus (the more stringent Tum'ah, which requires seven clean days and a Korban) from the same location as she is Metamei Nidus (the less stringent Tum'ah), so too is a Zav Metamei be'Zivus from the same location as he is Metamei be'Keri.
Why does the Torah add the word "Zovo"?
Rashi: To teach us that, not only is the Zav Tamei, but so is the Zivus that he emits. 1
See Sifsei Chachamim.
What is the difference between Zivus and Keri?
Rashi: Zivus is colorless, like the water of a barley-dough, and flows like the white of an egg that was formed without a male; whereas Keri is bound like the white of an egg that was formed from a male.
Why does the Torah mention 'Zivus' twice in this Pasuk ("Zav mi'Besaro" and "Zovo Tamei hu") and three times in the following Pasuk ("be'Zovo", "mi'Zovo" and "Zovo"), and call them both Tamei?
Rashi: To teach us that, although a man becomes a Zav after two sightings, 1 he only becomes a full-fledged Zav who is obligated to bring a Korban, after three sightings.
What is the reason for Tum'as Zov of a man?
Ramban (on Pasuk 11): Because it is a serious, contagious illness which comes on account of a man's sins. That is why he brings a Chatas to complete the atonement for his sins and an Olah to thank Hashem for healing him. 1
Seforno: It comes due to excessive intimacy and thinking about intimacy, which explains why a Zav requires seven clean days - to remove such thoughts from his mind. 2
Why does he bring an Olah, and not a Todah? One who brings Todah usually must invite others in order to consume the animal and 36 large loaves in one day, to avoid Nosar. Perhaps this would be embarrassing for the Zav. (PF)
If he had thoughts [about intimacy in the last 24 hours], he does not become a Zav (Tosefta Zavim 2:6)! Seforno must mean that previously he had excessive thoughts, and this leads to Zivah, even if he ceased having such thoughts for a day. (PF)