Exodus 4:6

His hand was leprous as snow - That is, the leprosy spread itself over the whole body in thin white scales; and from this appearance it has its Greek name λεπρα, from λεπις, a scale. Dr. Mead says, "I have seen a remarkable case of this in a countryman, whose whole body was so miserably seized with it, that his skin was shining as if covered with snow; and as the surfuraceous scales were daily rubbed off, the flesh appeared quick or raw underneath." The leprosy, at least among the Jews, was a most inveterate and contagious disorder, and deemed by them incurable. Among the heathens it was considered as inflicted by their gods, and it was supposed that they alone could remove it. It is certain that a similar belief prevailed among the Israelites; hence, when the king of Syria sent his general Naaman, to the king of Israel to cure him of his leprosy, he rent his clothes, saying, Amos I God, to kill and to make alive, that this man doth send unto me to recover a man of his leprosy? 2 Kings 5:7. This appears, therefore, to be the reason why God chose this sign, as the instantaneous infliction and removal of this disease were demonstrations which all would allow of the sovereign power of God. We need, therefore, seek for no other reasons for this miracle: the sole reason is sufficiently obvious.

Leprous - The instantaneous production and cure of the most malignant and subtle disease known to the Israelites was a sign of their danger if they resisted the command, and of their deliverance if they obeyed it. The infliction and cure were always regarded as special proofs of a divine intervention.

4:6 His hand was leprous, as snow - For whiteness. This signified, That Moses, by the power of God, should bring sore diseases upon Egypt, that at his prayer they should be removed. And that whereas the Israelites in Egypt were become leprous, polluted by sin, and almost consumed by oppression, by being taken into the bosom of Moses they should be cleansed and cured.