If any man among you becomes unclean because of a nocturnal emission, he must leave the camp and stay outside.
10. If there be among you. He enumerates two kinds of pollution, whereby the Israelites may know what is meant by their keeping from the “wicked thing.” First, He pronounces to be unclean, and casts out of the camp those who may have had a filthy dream, until they shall have washed themselves in the evening.
Israel was now encamped, and this vast army was just entering upon action, which was likely to keep them together for a long time, and therefore it was fit to give them particular directions for the good ordering of their camp. And the charge is in one word to be clean. They must take care to keep their camp pure from moral, ceremonial, and natural pollution. I.
Commenting on Deuteronomy 23:9-14
If there be among you any man that is not clean,.... Any unclean person in the army, that was even ceremonially unclean in any of the instances the law makes so, one of which put for the rest is mentioned: by reason of uncleanness that chanceth him by night; through pollution by a nocturnal flux, as the Septuagint version, or a gonorrhoea, an involuntary one...