Numbers 19:22

The unclean person; not he who is so only by touching the water of separation, Numbers 19:21, but he who is so by the greater sort of uncleanness, which lasted seven days, of which Numbers 19:11,Numbers 19:16, and which was not removed without the use of this water of purification, as is manifest from the context and other places of Scripture, and from the nature of the thing; for the lesser sort of uncleanness, mentioned in the foregoing verse, lasted only till even, and was cleansed by the sole washing of his clothes and flesh in water, Numbers 19:7, without any use of this water of purification, which, if it had been necessary, must have been used on the third and on the seventh day, according to the rule, Numbers 19:12,Numbers 19:19, and so the uncleanness of one day had been made an uncleanness of seven days, which is a contradiction. Besides it is unreasonable, that he who immediately touched the defiling thing, should be no more and longer unclean than he who touched that person only; and it was contrary to other rules in like cases, as Leviticus 15:0, where the man or woman having an issue, are thereby made unclean for seven days, Leviticus 15:13,Leviticus 15:28, but he who toucheth them is made unclean only till even, Leviticus 15:7,Leviticus 15:27. And therefore this cannot be meant of him who was unclean by touching this water, who himself was unclean only till even, Numbers 19:21, as also he who toucheth him is in this place.

Shall be unclean, to signify to us the very infectious nature of sin and of sinful company.

Until even, because as his defilement was less, so it was fit the duration of it should be shorter.