shalom
lama rav ashi medame zav le oved cochavim
haya yoter matim ledamot le tame met, mekevan shehazav
metame be medrass ma shen ken tame met.
yorenu hamore
avraham battat - brazil
(TRANSLATION:
Why does Rav Ashi compare Zav to Oved Kochavim? It would have been more appropriate to compare him to Tamei Mes, since the Zav is Metamei through Midras, while a Mes is not.)
Kavanas ha'Gemara l'hagdir mah nechshav "Kocho" shel Akum, u'mipnei zeh medamahu l'Zav, she'gam b'Zav tzerichim la'dun mah nechshav "Kocho" (u'metamei b'Heset), u'mah eino "Kocho." Aval b'Mes, lo danu bo mishum "Kocho" klal, v'ein ta'am l'hazkiro ba'sugya kan.
M. Kornfeld
(TRANSLATION:
The intention of the Gemara is to delineate what is considered "Kocho" of a Nochri, and therefore it compares him to a Zav, because we also must determine what is considered "Kocho" (and is Metamei through carrying) and what is not considered "Kocho" with regard to a Zav. However, with regard to a Mes, we do not discuss "Kocho" at all, and there is no reason to mention it in our Sugya.)