Sometimes it mentions the noun נגע (plague), and sometimes the noun צרעת (leprosy), and sometimes נגע צרעת (plague of leprosy). And there is a rule in this:

The noun נגע indicates the diseases that produce white hair, and thus this noun is mentioned in the chapter which talks about white hair.

And the noun צרעת indicates the diseases that produce raw flesh and is mentioned in the chapter which talks about raw flesh.

And thus, the noun נגע refers to a bright spot in that its way is to produce white hair, while the noun צרעת refers to a rising in that its way is to produce raw flesh.

And any place where it is not this way and it mentions the noun צרעת instead of the noun נגע – it comes to teach that its law is like the law for raw flesh, that its size must be at least the size of a split bean.