Adam Clarke on Leviticus 5:2

Any unclean thing - Either the dead body of a clean animal, or the living or dead carcass of any unclean creature. All such persons were to wash their clothes and themselves in clean water, and were considered as unclean till the evening, Lev 11:24-31. But if this had been neglected, they were obliged to bring a trespass-offering. What this meant, see in the notes on Leviticus 7 (note).