Understanding the kehati on Beitza 2:10

Question

Beitza 2:10:

עֲגָלָה שֶׁל קָטָן טְמֵאָה מִדְרָס וְנִטֶּלֶת בְּשַׁבָּת, וְאֵינָהּ נִגְרֶרֶת אֶלָּא עַל גַּבֵּי כֵלִים. רַבִּי יְהוּדָה אוֹמֵר, כָּל הַכֵּלִים אֵין נִגְרָרִין חוּץ מִן הָעֲגָלָה, מִפְּנֵי שֶׁהִיא כוֹבֶשֶׁת:

A child’s cart is susceptible to the impurity of midras, and it may be handled on Shabbat, and it is dragged along only on matting. Rabbi Judah says: no vessels may be dragged along [the ground] except a wagon because it [only] presses [the earth] down.

Kehati explains what a “child’s cart” is:

עגלה של קטן- העשויה לו כדי שילמד ללכת, והקטן נשען עליה והיא מתגלגלת לפניו. A child’s cart- that is made to teach him to walk, and the child leans on it and it rolls before him.

So our mishna is talking about a very young child- one that can’t even walk unaided.

Kehati also explains how the cart would become tamei:

טמאה מדרס- אם הקטן היה זב, הריהו מטמא את העגלה טומאת מדרס...

susceptible to the impurity of midras- if the child was a zav, he makes the cart tamei midras...

How can a young kid be a zav? Isn’t that medically impossible?

Answer 1

A boy can become impure as a zav from one day old, and a girl from ten days old (Nidda 5:3). I don't know about the medical aspect, but the Talmud (Nidda 32a) does mention cases of a baby girl seeing blood, which implies that she could become a zava, and if she were a zava she would be equally capable of making the cart impure in the case of the Mishna in Chagiga.