The Kli Yakar [13:2] says that this is inherently contained in the word metzorah itself. He explains that metzora is a combination of two words: ‘motzee’ – to take out, and ‘ra’ – evil. In other words, it reveals in an external – physical – manner the evil which had been lurking inside the person.
When it is all out and the Tzoraas begins to lessen on the person The Kohen declares him pure and it is clear the evil has gone hopefully never to come back again!