Sforno on Leviticus 13:3/ספורנו על ויקרא י״ג:ג׳

1נגע צרעת היא. The Torah repeats this same expression twice in successive verses, and also the expression צרעת היא is repeated both in verse 8 and verse 11. Sometimes, as here and as in verse 22, the word נגע is added. The reason may be that this affliction, in common with other afflictions, is something that develops in stages. Just as the affliction intensifies gradually in its early stages, so it gradually becomes weaker on the way to healing. When it is already a long-standing affliction it is referred to as צרעת יושנת, whereas when it is in its opening stages it is called simply נגע. When it is a fully fledged affliction it is called נגע צרעת. When it is in the process of waning it is already referred to as נרפא הצרעת, an affliction which has healed. When it has healed completely it is referred to as נרפא הנתק. Seeing that this type of affliction is perceived as a warning signal from G’d to the individual experiencing it, i.e. he is reminded to get rid of negative characteristics, the Talmud refers to it as מזבח כפרה “an altar serving as atonement.” The period of isolation serves to remind the afflicted person to review what he may have been guilty of. This concept has been spelled out in Job 36,10 as ויגל אותם למוסר ויאמר כי ישובון מאון, “He opens their understanding by discipline, and orders them back from mischief.”