Charles Ellicott Commentary Leviticus 13:3

"and the priest shall look on the plague in the skin of the flesh: and if the hair in the plague be turned white, and the appearance of the plague be deeper than the skin of his flesh, it is the plague of leprosy; and the priest shall look on him, and pronounce him unclean." — Leviticus 13:3 (ASV)

When the hair in the plague is turned white. —Better, and the hair in the plagued spot, etc. The first symptom indicating the existence of the disorder is that the hair, which is generally jet-black among the Hebrews, turns white on the affected spot. The authorities during the second Temple defined that there must be at least two hairs white, at the root and in the body of the bright spot, before the patient can be declared unclean. The word plague, in accordance with a usage common in Hebrew—to put the abstract for the concrete—denotes here the plagued spot, or the spot affected by the plague, while in Leviticus 13:4 it means the person affected by this disorder. Thus in Leviticus 19:32, the hoary head stands for hoary-headed person.

And the plague in sight be deeper than the skin. —Better, and the appearance of the plagued spot be deeper, etc. The second symptom which shows the development of the disorder is that the spot affected by this plague appears to be deeper than the rest of the skin.

Pronounce him unclean. —Literally, make him unclean. According to the frequently occurring phraseology, a man is said to do that which in his official capacity he pronounces as done, or orders to be done. Thus Ezekiel is said to destroy the city when he simply foretold its destruction (Ezekiel 43:3). The existence of these two symptoms made it incumbent upon the priest to declare the person unclean, and hence imparting defilement.