The Lord punished Miriam for her dissatisfaction with her divinely appointed role in the nation. He punished her with leprosy, the disease that specially symbolized sin ( Leviticus 13-14). Frank Cross suggested that Miriam"s punishment of white, leprous skin was a divine response to her prejudice against her black sister-in-law. [1] All the Israelites probably identified her self-assertion as sin. Perhaps God did not smite Aaron because his involvement was not as great. [source][source][source]
"In the present narrative, the sign of Moses" leadership was Miriam"s "leprosy, which was white as snow" ( Numbers 12:10). Similarly, one of the first signs given to vindicate God"s election of Moses as leader of his people was the sign of "leprosy, white as snow" ( Exodus 4:6). In the initial narratives dealing with the work of Moses, Moses himself doubted his calling and consequently became a leper. Here, however, it is Miriam who doubts and thus becomes a leper. We should also note that the other sign given to vindicate the role of Moses in the earlier narrative was the serpent that came from Moses" rod ( Exodus 4:3). So also here, when Moses" authority is further questioned by the people at the end of their time in the wilderness ( Numbers 21:5), God responds by sending serpents against them ( Numbers 21:6)." [2][source]
Moses interceded for Miriam at Aaron"s pleading. Ironically Aaron had wanted to be like Moses ( Numbers 12:2), but instead of being able to intercede directly with God as Moses did, Aaron had to appeal to Moses who interceded with God. [3] God again showed mercy. He removed Miriam"s leprosy but punished her with exclusion from the camp for seven days ( Leviticus 14:8). Spitting in the face ( Numbers 12:14) was an act of contempt for one who had done something despicable (cf. Deuteronomy 25:9; Job 17:6; Job 30:10). The people suffered too as a result of Miriam and Aaron"s rebellion. God halted their progress toward the Promised Land again ( Numbers 12:15; cf. Numbers 11:20). [source][source][source]
"Aaron, on seeing the judgment of leprosy come upon his sister, beseeches Moses for mercy. There is surely in his prayer an implicit recognition of the different kind of authority that Moses had. Indeed, he is acknowledging that Moses possessed a power in intercession with God that he himself could not exercise, hence his appeal to his brother." [4][source]
"The purpose of this chapter, then, is to vindicate Moses" divinely given leadership and to brush aside any further suggestion that, because of the establishment of other forms of authority, the type of leadership epitomized in Moses was no longer valid." [5][source]