Yoma 30a and Yoma 30b, hygiene for after using the facilities, and then the mikveh immersion before entering the Azarah, whether it is a Biblical requirement regardless of removing ritual impurity, is a way to ensure the person remembers any ritual impurity, or is only for a recovered metzora, who until now has been lax about ritual impurity (because s/he was necessarily ritually impure by virtue of the tzara’at).
