Charles Ellicott Commentary Leviticus 5:3

"Or if he touch the uncleanness of man, whatsoever his uncleanness be wherewith he is unclean, and it be hid from him; when he knoweth of it, then he shall be guilty." — Leviticus 5:3 (ASV)

Or if he touch the uncleanness of man. — The various classes of defilement that a human being might contract and impart to others by contact are set forth in Leviticus 12-15.

When he knoweth of it, then he shall be guilty. — Better, and he knoweth it, and feeleth that he is guilty. That is, he afterwards becomes conscious that he has contracted the defilement, and feels his guilt. (See Leviticus 5:2.)