Numbers 5:1

The camps and divisions of priests, Levites, and people being thus settled, now was the time when the law about excluding leprous and unclean persons from the camps was to take place; God having, for wise reasons, appointed that all persons under such legal impurities should, in proportion to the degree of them, be excluded from the community where he himself dwelt by the symbols of his divine presence till they were cleansed again. This the Israelites began now to put in execution by express order from God to Moses.