Leviticus 13:8

And if the priest see that, behold, the scab spreadeth in the skin, then the priest shall pronounce him unclean: it is a leprosy.

Cross-reference

Leviticus 13:3 gives initial diagnosis criteria; here the priest declares unclean after spread — both are steps in the same examination process.

Leviticus 14:39 applies the same spreading test to mildew in a house — a parallel principle of contamination.