The garment also that the plague of leprosy is in, whether it be a woolen garment, or a linen garment;

Wesley's Notes

13:47 Leprosy in garments and houses is unknown in these times and places, which is not strange, there being some diseases peculiar to some ages and countries. And that such a thing was among the Jews, cannot reasonably be doubted; for, if Moses had been a deceiver, a man of his wisdom, would not have exposed himself to the contempt of his people by giving laws about that which their experience shewed to be but a fiction.

Jamieson-Fausset-Brown Bible Commentary

47-59. The 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 (Nu 12:10; 2Ki 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 archæology 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.

Matthew Henry's Concise Commentary

13:47-59 The garment suspected to be tainted with leprosy was not to be burned immediately. If, upon search, it was found that there was a leprous spot, it must be burned, or at least that part of it. If it proved to be free, it must be washed, and then might be used. This also sets forth the great evil there is in sin. It not only defiles the sinner's conscience, but it brings a stain upon all he has and all that he does. And those who make their clothes servants to their pride and lust, may see them thereby tainted with leprosy. But the robes of righteousness never fret, nor are moth-eaten.