A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown [1882] at sacred-texts.com
When any man hath a running issue--This chapter describes other forms of uncleanness, the nature of which is sufficiently intelligible in the text without any explanatory comment. Being the effects of licentiousness, they properly come within the notice of the legislator, and the very stringent rules here prescribed, both for the separation of the person diseased and for avoiding contamination from anything connected with him, were well calculated not only to prevent contagion, but to discourage the excesses of licentious indulgence.
what saddle . . . he rideth upon that hath the issue shall be unclean--(See on Gen 31:34).
the vessel of earth, that he toucheth which hath the issue, shall be broken--It is thought that the pottery of the Israelites, like the earthenware jars in which the Egyptians kept their water, was unglazed and consequently porous, and that it was its porousness which, rendering it extremely liable to imbibe small particles of impure matter, was the reason why the vessel touched by an unclean person was ordered to be broken.
then he shall number to himself seven days for his cleansing--Like a leprous person he underwent a week's probation, to make sure he was completely healed. Then with the sacrifices prescribed, the priest made an atonement for him, that is, offered the oblations necessary for the removal of his ceremonial defilement, as well as the typical pardon of his sins.
UNCLEANNESS OF WOMEN. (Lev 15:19-33)
if a woman have an issue--Though this, like the leprosy, might be a natural affection, it was anciently considered contagious and entailed a ceremonial defilement which typified a moral impurity. This ceremonial defilement had to be removed by an appointed method of ceremonial expiation, and the neglect of it subjected any one to the guilt of defiling the tabernacle, and to death as the penalty of profane temerity.
Thus shall ye separate the children of Israel from their uncleanness--The divine wisdom was manifested in inspiring the Israelites with a profound reverence for holy things; and nothing was more suited to this purpose than to debar from the tabernacle all who were polluted by any kind of uncleanness, ceremonial as well as natural, mental as well as physical. The better to mark out that people as His family, His servants and priests, dwelling in the camp as in a holy place, consecrated by His presence and His tabernacle, He required of them complete purity, and did not allow them to come before Him when defiled, even by involuntary or secret impurities, as a want of respect due to His majesty. And when we bear in mind that God was training a people to live in His presence in some measure as priests devoted to His service, we shall not consider these rules for the maintenance of personal purity either too stringent or too minute (Th1 4:4).