Komentarz do Kapłańska 15:8

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).

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.