Sforno on Leviticus 17:2/ספורנו על ויקרא י״ז:ב׳

1זה הדבר, when the Torah said in 15,31 והזהרתם את בני ישראל מטומאתם, “you are to warn the Israelites regarding their state of ritual contamination,” the warning implied that they were to separate themselves from spiritually negative influences represented both by ritual contamination and by demons.