פירוש על ויקרא 13:8

וטמאו הכהן AND THE PRIEST SHALL PRONOUNCE HIM UNCLEAN — and as soon as he has pronounced him unclean he is a decided leper and he is obliged when he recovers from his leprosy to bring two birds (as a means of his purification) and to shave his flesh and to bring the offering which is mentioned in the section beginning זאת תהיה — Leviticus ch. 14 (cf. Megillah 8b).

צרעת הוא. it is a form of leprosy. The meaning of these words is that even if the priest had previously declared the person afflicted as "clean," this judgment is reversesd retroactively in light of the afflicted area having spread. The reason is that a scab which does not indicate ritual impurity is not in the habit of spreading. The Torah indicates that the spreading needs only to be minimal in order for the afflicted person to be declared as ritually impure. It is the scab which is the affliction, not its size.

צרעת היא, definitive, beyond doubt. The afflicted person has to remain in isolation until cured when he or she will offer bird-offerings.