A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
lev 13:2THE LAWS AND TOKENS IN DISCERNING LEPROSY. (Lev. 13:1-59)
When a man shall have in the skin, &c.--The fact of the following rules for distinguishing the plague of leprosy being incorporated with the Hebrew code of laws, proves the existence of the odious disease among that people. But a short time, little more than a year (if so long a period had elapsed since the exodus) when symptoms of leprosy seem extensively to have appeared among them; and as they could not be very liable to such a cutaneous disorder amid their active journeyings and in the dry open air of Arabia, the seeds of the disorder must have been laid in Egypt, where it has always been endemic. There is every reason to believe that this was the case: that the leprosy was not a family complaint, hereditary among the Hebrews, but that they got it from intercourse with the Egyptians and from the unfavorable circumstances of their condition in the house of bondage. The great excitement and irritability of the skin in the hot and sandy regions of the East produce a far greater predisposition to leprosy of all kinds than in cooler temperatures; and cracks or blotches, inflammations or even contusions of the skin, very often lead to these in Arabia and Palestine, to some extent, but particularly in Egypt. Besides, the subjugated and distressed state of the Hebrews in the latter country, and the nature of their employment, must have rendered them very liable to this as well as to various other blemishes and misaffections of the skin; in the production of which there are no causes more active or powerful than a depressed state of body and mind, hard labor under a burning sun, the body constantly covered with the excoriating dust of brick fields, and an impoverished diet--to all of which the Israelites were exposed while under the Egyptian bondage. It appears that, in consequence of these hardships, there was, even after they had left Egypt, a general predisposition among the Hebrews to the contagious forms of leprosy--so that it often occurred as a consequence of various other affections of the skin. And hence all cutaneous blemishes or blains--especially such as had a tendency to terminate in leprosy--were watched with a jealous eye from the first [GOOD, Study of Medicine]. A swelling, a pimple, or bright spot on the skin, created a strong ground of suspicion of a man's being attacked by the dreaded disease.
then he shall be brought unto Aaron the priest, &c.--Like the Egyptian priests, the Levites united the character of physician with that of the sacred office; and on the appearance of any suspicious eruptions on the skin, the person having these was brought before the priest--not, however, to receive medical treatment, though it is not improbable that some purifying remedies might be prescribed, but to be examined with a view to those sanitary precautions which it belonged to legislation to adopt.
lev 13:3the priest shall look on the plague in the skin of the flesh, &c.--The leprosy, as covering the person with a white, scaly scurf, has always been accounted an offensive blemish rather than a serious malady in the East, unless when it assumed its less common and malignant forms. When a Hebrew priest, after a careful inspection, discovered under the cutaneous blemish the distinctive signs of contagious leprosy, the person was immediately pronounced unclean, and is supposed to have been sent out of the camp to a lazaretto provided for that purpose. If the symptoms appeared to be doubtful, he ordered the person to be kept in domestic confinement for seven days, when he was subjected to a second examination; and if during the previous week the eruption had subsided or appeared to be harmless, he was instantly discharged. But if the eruption continued unabated and still doubtful, he was put under surveillance another week; at the end of which the character of the disorder never failed to manifest itself, and he was either doomed to perpetual exclusion from society or allowed to go at large. A person who had thus been detained on suspicion, when at length set at liberty, was obliged to "wash his clothes," as having been tainted by ceremonial pollution; and the purification through which he was required to go was, in the spirit of the Mosaic dispensation, symbolical of that inward purity it was instituted to promote.
lev 13:7But if the scab spread much abroad in the skin--Those doubtful cases, when they assumed a malignant character, appeared in one of two forms, apparently according to the particular constitution of the skin or of the habit generally. The one was "somewhat dark" [Lev 13:6] --that is, the obscure or dusky leprosy, in which the natural color of the hair (which in Egypt and Palestine is black) is not changed, as is repeatedly said in the sacred code, nor is there any depression in the dusky spot, while the patches, instead of keeping stationary to their first size, are perpetually enlarging their boundary. The patient laboring under this form was pronounced unclean by the Hebrew priest or physician, and hereby sentenced to a separation from his family and friends--a decisive proof of its being contagious.
lev 13:9if the rising be white--This BRIGHT WHITE leprosy is the most malignant and inveterate of all the varieties the disease exhibits, and it was marked by the following distinctive signs: A glossy white and spreading scale, upon an elevated base, the elevation depressed in the middle, but without a change of color; the black hair on the patches participating in the whiteness, and the scaly patches themselves perpetually enlarging their boundary. Several of these characteristics, taken separately, belong to other blemishes of the skin as well; so that none of them was to be taken alone, and it was only when the whole of them concurred that the Jewish priest, in his capacity of physician, was to pronounce the disease a malignant leprosy. If it spread over the entire frame without producing any ulceration, it lost its contagious power by degrees; or, in other words, it ran through its course and exhausted itself. In that case, there being no longer any fear of further evil, either to the individual himself or to the community, the patient was declared clean by the priest, while the dry scales were yet upon him, and restored to society. If, on the contrary, the patches ulcerated and quick or fungous flesh sprang up in them, the purulent matter of which, if brought into contact with the skin of other persons, would be taken into the constitution by means of absorbent vessels, the priest was at once to pronounce it an inveterate leprosy. A temporary confinement was them declared to be totally unnecessary, and he was regarded as unclean for life [DR. GOOD]. Other skin affections, which had a tendency to terminate in leprosy, though they were not decided symptoms when alone, were: "a boil" (Lev 13:18-23); "a hot burning,"--that is, a fiery inflammation or carbuncle (Lev 13:24-28); and "a dry scall" (Lev 13:29-37), when the leprosy was distinguished by being deeper than the skin and the hair became thin and yellow.
lev 13:38If a man . . . or a woman have in the skin of their flesh bright spots--This modification of the leprosy is distinguished by a dull white color, and it is entirely a cutaneous disorder, never injuring the constitution. It is described as not penetrating below the skin of the flesh and as not rendering necessary an exclusion from society. It is evident, then, that this common form of leprosy is not contagious; otherwise Moses would have prescribed as strict a quarantine in this as in the other cases. And hereby we see the great superiority of the Mosaic law (which so accurately distinguished the characteristics of the leprosy and preserved to society the services of those who were laboring under the uncontagious forms of the disease) over the customs and regulations of Eastern countries in the present day, where all lepers are indiscriminately proscribed and are avoided as unfit for free intercourse with their fellow men.
lev 13:40bald . . . forehead bald--The falling off of the hair, when the baldness commences in the back part of the head, is another symptom which creates a suspicion of leprosy. But it was not of itself a decisive sign unless taken in connection with other tokens, such as a "sore of a reddish white color" [Lev 13:43]. The Hebrews as well as other Orientals were accustomed to distinguish between the forehead baldness, which might be natural, and that baldness which might be the consequence of disease.
lev 13:45the leper in whom the plague is, his clothes shall be rent, &c.--The person who was declared affected with the leprosy forthwith exhibited all the tokens of suffering from a heavy calamity. Rending garments and uncovering the head were common signs of mourning. As to "the putting a covering upon the upper lip," that means either wearing a moustache, as the Hebrews used to shave the upper lip [CALMET], or simply keeping a hand over it. All these external marks of grief were intended to proclaim, in addition to his own exclamation "Unclean!" that the person was a leper, whose company every one must shun.
lev 13:46he shall dwell alone; without the camp--in a lazaretto by himself, or associated with other lepers (Kg2 7:3, Kg2 7:8).
lev 13:47The garment . . . that the . . . leprosy is in--It is well known that infectious diseases, such as scarlet fever, measles, the plague, are latently imbibed and carried by the clothes. But the language of this passage clearly indicates a disease to which clothes themselves were subject, and which was followed by effects on them analogous to those which malignant leprosy produces on the human body--for similar regulations were made for the rigid inspection of suspected garments by a priest as for the examination of a leprous person. It has long been conjectured and recently ascertained by the use of a lens, that the leprous condition of swine is produced by myriads of minute insects engendered in their skin; and regarding all leprosy as of the same nature, it is thought that this affords a sufficient reason for the injunction in the Mosaic law to destroy the clothes in which the disease, after careful observation, seemed to manifest itself. Clothes are sometimes seen contaminated by this disease in the West Indies and the southern parts of America [WHITLAW, Code of Health]; and it may be presumed that, as the Hebrews were living in the desert where they had not the convenience of frequent changes and washing, the clothes they wore and the skin mats on which they lay, would be apt to breed infectious vermin, which, being settled in the stuff, would imperceptibly gnaw it and leave stains similar to those described by Moses. It is well known that the wool of sheep dying of disease, if it had not been shorn from the animal while living, and also skins, if not thoroughly prepared by scouring, are liable to the effects described in this passage. The stains are described as of a greenish or reddish color, according, perhaps, to the color or nature of the ingredients used in preparing them; for acids convert blue vegetable colors into red and alkalis change then into green [BROWN]. It appears, then, that the leprosy, though sometimes inflicted as a miraculous judgment (Num 12:10; Kg2 5:27) was a natural disease, which is known in Eastern countries still; while the rules prescribed by the Hebrew legislator for distinguishing the true character and varieties of the disease and which are far superior to the method of treatment now followed in those regions, show the divine wisdom by which he was guided. Doubtless the origin of the disease is owing to some latent causes in nature; and perhaps a more extended acquaintance with the archÃ&brvbr;ology of Egypt and the natural history of the adjacent countries, may confirm the opinion that leprosy results from noxious insects or a putrid fermentation. But whatever the origin or cause of the disease, the laws enacted by divine authority regarding it, while they pointed in the first instance to sanitary ends, were at the same time intended, by stimulating to carefulness against ceremonial defilement, to foster a spirit of religious fear and inward purity.