וַיֹּ֣אמֶר חַגַּ֔י אִם־יִגַּ֧ע טְמֵא־נֶ֛פֶשׁ בְּכָל־אֵ֖לֶּה הֲיִטְמָ֑א וַיַּעֲנ֧וּ הַכֹּהֲנִ֛ים וַיֹּאמְר֖וּ יִטְמָֽא׃
Rashi on Haggai
It will become contaminated For the dead body is the father of the father of contamination. The fifth degree of the contamination of the reptile or rodent is a fourth degree in the contamination of a dead body.