Leviticus 14:4

Later usage required the birds to be sparrows. Cedar wood (probably not the cedar of Lebanon but a kind of juniper) may have been chosen on account of its antiseptic property, and hyssop (see on Exodus 12:22) for its aromatic qualities. In later times, at least, their use was regarded as symbolical, in the one case of the pride which was supposed to be the cause of visitation by the disease, in the other of the humility which was an essential condition of its removal. The scarlet wool may have betokened the healthy blood now coursing in the veins of the erewhile leper. The same materials were employed in the ritual for purification after contact with dead bodies: see Numbers 19:6 and cp. Psalms 51:7.