The whole chapter of Numbers 12 deals with the incident where Aaron and Miriam speak against Moses because of his Cushite wife. She was a black woman who had presumably attached herself (or her family attached themselves) to the Israelites after witnessing the contest between the Lord and the Egyptians.

Here is a study of the biblical Cushites: http://www.asor.org/anetoday/2020/12/cushites-hebrew-bible

Miriam was struck with a condition which made her skin a sickly white. Why Aaron was not struck with this malady may have been due to his position as Israel’s High Priest. Alternately, it may have been that Miriam, the oldest sibling, was the instigator. Regardless, Moses prays for his sister to be healed and she is restored immediately. We know the healing is instantaneous since the quarantine requirement of seven days is after a person is symptom free (see Lev. 13. 1-5).