Rashi on Leviticus 15:8/רש"י על ויקרא ט״ו:ח׳

1וכי 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).