וְכִֽי־יָרֹ֛ק הַזָּ֖ב בַּטָּה֑וֹר וְכִבֶּ֧ס בְּגָדָ֛יו וְרָחַ֥ץ בַּמַּ֖יִם וְטָמֵ֥א עַד־הָעָֽרֶב׃
At kapag ang may agos ay dumura sa taong malinis, maglalaba ang taong iyon ng kanyang mga kasuotan at maliligo sa tubig, at magiging marumi hanggang sa gabi.
Rashi
וכי ירק הזב בטהור AND IF HE THAT HATH THE ISSUE SPIT UPON ONE WHO IS CLEAN — and he (the clean person) comes in contact with it (the spittle; i. e. it actually falls upon him) or even if he only bears it (i. e. it has, for instance, fallen upon a stick which he is carrying) then HE SHALL BE UNCLEAN UNTIL THE EVEN, for the spittle communicates uncleanness through carrying it (cf. Niddah 55b).
Ibn Ezra
If he spit an ‘ayin-‘ayin verb (witness the doubling of the qof in “my spittle” [Job 7:19]). Even his spit is harmful, because the disease from which he suffers is contagious. Although people do not customarily spit in front of one another, it can nonetheless happen unintentionally: a man with a flow spits, and it falls on someone who was ritually pure.
Siftei Chakhamim
Because spittle causes uncleanness by means of carrying. This is how it is derived: “If the zov spits on a ritually clean person (בטהור),” and it is not written, “upon (על) the ritually clean person,” i.e., [he spits] on what is in the hand of the pure person, that is, if he spits on the staff that is in the pure person’s hand who is carrying it, he becomes impure.