It is occurred when she and Aaron challenged God’s decision to elevate Moses: “But when the cloud had withdrawn from over the tent, behold, Miriam was leprous, as white as snow.
Moses is more devoted to God than any other person, and Miriam and Aaron are jealous of Moses' special relationship with God. God speaks to the three of them, scolding Miriam and Aaron, and when the cloud of God's glory is gone, Miriam has leprosy. That is, she has a skin disease that makes her ritually unclean. In horror, Aaron appeals to Moses to help them. Again, I don't know why Miriam was the only one who was punished. I don't know why Aaron didn't sit with her outside the camp for 7 days. But I do know that in Jesus Christ we have the power to overcome these inequalities for in Jesus Christ there is neither East nor West, Jew nor Greek, slave nor free, male nor female. Both Aaron and Miriam spoke disparagingly of Moses. Both failed to grasp the unique nature of their brother's prophecy; they thought Moses was their spiritual and prophetic equal. God was angry with both of them, but only Miriam was punished with leprosy. God displayed anger with them, and departed. When the cloud left the Tent, Miriam was leprous, white like snow. Aaron turned to Miriam, and saw she was leprous. (Num. The exact nature of Aaron's punishment, however is a matter of dispute. Rabbi Akiva said that Aaron also suffered from leprosy. But, unlike Miriam who was punished for a full week, Aaron's affliction was transient. Aaron, the younger brother to Miriam and Moses went along with Miriam's public criticism of Moses and his new wife Zipporah. Almighty God heard Miriam's false and petty accusations against Moses, and Aaron support of what she said. But even as Almighty God punished her instantly with full blown, last stage leprosy, because of Aaron's contrition and intercession on her behalf and Moses willingness to also intercede on his foolish sister's behalf, Almighty God did heal Miriam after 8 days. God asked Aaron and Miriam. Then God got really mad at Aaron and Miriam, so he left. When the cloud was gone, Miriam got leprosy. Aaron looked at her and said, Look! She's got leprosy!. Aaron said to Moses, Hey, I'm sorry. Please, don't punish us because we're stupid When the cloud lifted from above the tent, Miriam's skin was leprous --it became as white as snow. Aaron turned toward her and saw that she had a defiling skin disease, New Living Translation. As the cloud moved from above the Tabernacle, there stood Miriam, her skin as white as snow from leprosy. And behold, Miriam was leprous as snow; and Aaron turned toward Miriam, and behold, she was leprous. English Revised Version. And the cloud removed from over the Tent; and, behold, Miriam was leprous, as white as snow: and Aaron looked upon Miriam, and, God was angry with them, and punished Miriam with leprosy.  God displayed anger with them and departed. When the cloud left the Tent, Miriam was leprous, white like snow. Aaron turned to Miriam, and saw she was leprous. (Num.  Numbers 12:1,9-15 Then Miriam and Aaron spoke against Moses because of the Cushite woman whom he had married (for he had married a Cushite woman).So the anger of the LORD burned against them and He departed. But when the cloud had withdrawn from over the tent, behold, Miriam was leprous, as white as snow.  Miriam struck with leprosy, and healed at the prayer of Moses. The cloud departed, and Miriam became leprous. When God goes, evil comes: expect no good when God departs. Her foul tongue, as Bishop Hall says, was justly punished with a foul face. Aaron, as priest, was judge of the leprosy. He could not pronounce her leprous without trembling, knowing himself to be equally guilty In this story, Miriam is stricken with leprosy, punished by God, for doing the EXACT same thing that Aaron does—complain about Moses. Aaron is not punished and in fact, goes to plead to Moses on Miriam’s behalf. God yells at Miriam and Aaron, storms off, and Miriam is left with leprosy and has skin as white as snow (which, if she’s being punished for being racist, would be quite appropriate). Aaron apologizes to Moses for both himself and Miriam, wherein he acknowledges that he sinned as did she. The incident of Miriam’s leprosy occurred when she and Aaron challenged God’s decision to elevate Moses: “But when the cloud had withdrawn from over the tent, behold, Miriam was leprous, as white as snow. As Aaron turned toward Miriam, behold, she was leprous. 
http://www.dpchurch.com/node/142 https://answers.yahoo.com/question/index?qid=20100528103221AAJQcUk https://answers.yahoo.com/question/index?qid=20080122175607AA2HV3y http://www.alltruebible.com/numbers_12-01.shtml http://biblehub.com/numbers/12-10.htm http://www.israelnationalnews.com/Articles/Article.aspx/11768 http://literalbible.blogspot.com/2007/03/god-curses-miriam-with-leprosy-for.html https://www.biblegateway.com/resources/commentaries/Matthew-Henry/Num/Miriam-Struck-Leprosy-Healed http://www.the-exponent.com/women-in-the-scriptures-why-does-god-hate-miriam/ https://3amthoughts.com/article/people-and-places/miriams-leprosy
why did only miriam get leprosy