Refer to Avot D'Rebbi Natan ch. 9 (ver. א. In summary, he comments on Numbers 14:22: surely all those men that have seen My glory, and My signs, which I wrought in Egypt and in the wilderness, ...
When the Chumash speaks about a kohen's seeing some tzaraas on a person's body for the first time, it says the kohen "shall defile" (Vayikra 13:3, 13:11, 13:20, 13:25, 13:30, 13:44). Rashi (13:3) ...