Leviticus 13:1

And the Lord spake with Mosheh, saying:

God told Moshe and Aaron Why Aaron? Because his pronouncements would determine, for all the afflicted of Israel, who shall be ritually pure and who shall be ritually impure.

AND THE ETERNAL SPOKE UNTO MOSES AND UNTO AARON. Because it is according to the word of the priest that every strife and every plague shall be,). — It should also be pointed out that while tzara’ath is generally translated as “leprosy” and as such represents a physical disarray in the body of the person, in Jewish tradition it also carries with it a moral and spiritual meaning as illumined further on by Ramban in Verse 47. therefore this communication came also to Aaron. Or it may mean that G-d spoke to Moses that he should tell it to Aaron, as our Rabbis have explained. It does not state here, “speak unto the children of Israel,” because it is the priests [who have the duty] when they see the impure [with leprosy] to force them to be quarantined and be cleansed. Now in the section dealing with cleansing of the leper it says, And the Eternal spoke unto Moses, saying: This shall be the law of the leper in the day of his cleansing, [and there it mentions neither “speak to the children of Israel,” nor to the priests], because there is no need to urge the Israelite to become cleansed [when his plague of leprosy is healed], nor to urge the priest to perform the rites of the offerings, as they do so willingly. In the section dealing with a person suffering from an issue it does say, Speak unto the children of Israel, and say unto them,, 15:2. because since the matter is of an intimate nature, unknown to others, He admonished them that they should each inform the priest of their sickness. dependent upon the pronouncement of the priest, unlike the law in cases of leprosy (see Note 46 above), Ramban’s words must be understood in the following sense: Since the priest has occasion to perform the Service in the Sanctuary, and even when he is outside it, he may eat terumah (the heave-offering), which he is forbidden to eat when impure, therefore the Israelite who is suffering from a flux and thus rendered impure must let the priest know of his condition, so that he should not be defiled on account of him. See my Hebrew commentary, Vol. II, p. 534 (beginning with the fifth edition).