Charles Ellicott Commentary Leviticus 15:11

"And whomsoever he that hath the issue toucheth, without having rinsed his hands in water, he shall wash his clothes, and bathe himself in water, and be unclean until the even." — Leviticus 15:11 (ASV)

And hath not rinsed his hands in water. — If the patient happens to touch anyone with his hands without having first washed his hands, the man who has thus been touched by the unwashed hands of the defiled patient contracts pollution until sundown of the day on which he has been touched. He has to wash his clothes and immerse his whole body in water before he can partake of the privileges of the sanctuary.

This is the only instance where the touch of the hand as imparting defilement is expressly mentioned, and where the washing of the hands alone is ordered in the Mosaic Law to prevent the communication of pollution. The washing of the hands over the heifer, ordered in Deuteronomy 21:6, is of a different kind. It is meant to renounce any share in the guilt of the murder, or rather, to protest their innocence.