Charles Spurgeon Commentary Numbers 19:17-22

"And for the unclean they shall take of the ashes of the burning of the sin-offering; and running water shall be put thereto in a vessel: and a clean person shall take hyssop, and dip it in the water, and sprinkle it upon the tent, and upon all the vessels, and upon the persons that were there, and upon him that touched the bone, or the slain, or the dead, or the grave: and the clean person shall sprinkle upon the unclean on the third day, and on the seventh day: and on the seventh day he shall purify him; and he shall wash his clothes, and bathe himself in water, and shall be clean at even. But the man that shall be unclean, and shall not purify himself, that soul shall be cut off from the midst of the assembly, because he hath defiled the sanctuary of Jehovah: the water for impurity hath not been sprinkled upon him; he is unclean. And it shall be a perpetual statute unto them: and he that sprinkleth the water for impurity shall wash his clothes, and he that toucheth the water for impurity shall be unclean until even. And whatsoever the unclean person toucheth shall be unclean; and the soul that toucheth it shall be unclean until even." — Numbers 19:17-22 (ASV)

This ordinance was partly sanitary. The Egyptians were accustomed to keep their dead in their houses, preserved as mummies. No Jew could do that, because he would be defiled. Other nations were accustomed to bury their dead, as we once did, within the city walls, or around their own places of worship, as if to bring death as close as they could to themselves. No Jew could do this, because he was defiled if he even passed over a grave; so they were driven to what God intended they should have—that is, extramural interments, and to keep the graveyard as far as they could away from the homes of the living.

The spiritual meaning of this regulation is that we must watch with great care against every occasion for sin; and, since there will be these occasions and we shall be defiled, we must constantly go to the Lord with a prayer like that of David in Psalm 51, which we will now read.