Or HaChaim on Leviticus 13:8/אור החיים על ויקרא י״ג:ח׳

1 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.