ויקרא, פרק י״ג, פסוק ח׳

When a skin condition that was previously examined and cleared suddenly begins to grow, the medical and spiritual status of the individual dramatically shifts. The priest must re-evaluate the situation, and this new growth often leads to a declaration of absolute impurity.

This expansion refers to a situation where the mark grows beyond its original borders after the person was already declared pure [רלב״ג]. However, not every expansion automatically results in impurity. For the new growth to render the person impure, it must appear in one of the four specific shades of impurity, similar to the original mark's color. If the mark spreads but the new area takes on a pure color, the person remains pure [תורה תמימה, מלבי״ם, אדרת אליהו]. On the other hand, the new growth does not need to meet the minimum size requirement of a standard affliction. Even the slightest expansion is enough to establish impurity, because a genuinely pure mark naturally stays exactly as it is without spreading at all [אור החיים].

Upon seeing this expansion, the priest officially declares the impurity. Some explain that the priest's declaration is directed at the mark itself, legally confirming its impure status [רד צ הופמן]. This ruling fundamentally alters the individual's standing, turning them into a confirmed leper. Most commentators emphasize a profound difference between someone who is merely placed in a temporary quarantine and someone who is officially confirmed as impure. While both share an impure status, their paths to purification are completely different. A person leaving temporary quarantine simply needs to wash their clothes and immerse in water. In stark contrast, a confirmed leper must undergo a long and complex rehabilitation process, which includes living in isolation, bringing birds, shaving all body hair, and offering special sacrifices [רש״י, רשב״ם, מזרחי, גור אריה, משכיל לדוד].

The final determination emphasizes that this specific mark is indeed leprosy. Because the initial term used often describes a harmless scab, it is clarified that this particular affliction—which initially seemed completely pure—is now retroactively exposed as actual, impure leprosy [רש״י, מזרחי, גור אריה, אור החיים].