Leviticus 13:2

In the skin, for there was the seat of the leprosy. Bright spot, shining like the scale of a fish, as it is in the beginning of a leprosy. Leprosy was a distemper most frequent in Egypt and Syria, &c., known also among the Greeks, who note that it was not so properly a disease as a defilement or distemper in the skin, whence Christ is not said to heal, but to cleanse the lepers that came to him. And this distemper is here provided against, not because it was worse than others, but because it was externally and visibly filthy, and because of its infectious nature, that hereby we might be instructed to avoid converse with such vicious persons who were likely to infect us. He shall be brought unto Aaron the priest, not to the physician, because, as was now said, it needed not so much healing as cleansing, and was rather a ceremonial pollution than a disease; and because it belonged to the priest to cleanse him, and therefore to search and discover whether he was defiled and needed cleansing. The priest also was to admit to, or exclude from, the sanctuary, and therefore to examine who were to be excluded. And the discovery of this distemper was not so difficult that it required the physician's art, but the priest, by experience, and the observation of those rules, might easily make it.