So Miriam was shut up outside the camp for seven days, and the people did not move on until Miriam was received again. Numbers 12:15
"Even Aaron and Miriam were not above the sin of grumbling. Numbers 12:1-2 relates a few details about their complaint against Moses. Apparently they had something against Moses because of the Cushite woman he had married. (The Torah does not tell us the details of their gripe, but people are often irritated by their sibling's spouses.)
The complaint against Moses had to do with his role as leader over the assembly. Both Miriam and Aaron were prophets in their own right. They had both personally received prophecies from God. They began to resent Moses' sole leadership over the assembly. "Has the LORD indeed spoken only through Moses? Has He not spoken through us as well?" (Numbers 12:2), they asked. Miriam and Aaron assumed that no one could hear their private conversation. It was their own private gripe against their brother. They forgot that God could hear. The Torah says, "And the LORD heard it" (Numbers 12:2).
How many times do we indulge in similar "private" conversations, forgetting that God is listening?
The LORD struck Miriam with leprosy as a punishment for speaking evil speech against her brother. Moses immediately interceded on her behalf with a short, urgent prayer. The LORD relented and removed the leprosy, but Miriam still had to be put outside of the camp for seven days until she was ritually fit again.
The most puzzling thing about the story is why Miriam was smitten with leprosy while Aaron was not. Is that fair? Perhaps Aaron was spared because of his responsibility in the priesthood or perhaps it was that Miriam was punished more harshly because she was the instigator of the gossip. Those are possible explanations, but there seems to have been one ancient tradition that taught Aaron was also struck with leprosy.
According to that tradition, the Torah does not explicitly mention Aaron's punishment out of respect for the office of the high priest. Instead, Aaron's punishment was edited out of the record but remembered nonetheless.
We know this was an opinion in early Judaism because one ancient rabbi warns that "Anyone who says that Aaron was also smitten with leprosy will have to give an account [in heaven]. When God has concealed the matter concerning Aaron [how dare we reveal it?]" (Sifre 105).
Clement, the disciple of the Peter, preserved the same tradition. He taught that Aaron was also put out of the camp for seven days. Clement says, "On account of envy, Aaron and Miriam had to make their abode outside the camp" (1 Clement 4:11 )."
Adapted from aish and ffoz commentaries by Benzion HaLevi Sherry