Jamieson, Fausset and Brown on Leviticus 15:19

Le 15:19-33. Uncleanness of Women.

19. 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.