Tzuras is a disease only talmidei chachomim can get

 The Netziv in his commentatary to Tazria raises a number of interesting points 

He says based on the Zohar that the Adam mentioned in the parsha is the highest level human - a talmid chachom whose sinning is a chillul HaShem and thus requires exile from the camp - i.e. exile. An ordinary person doesn't get tzuras and that is why a cohen - not a sage decides if it is truly tzuras - by means of a form of prophecy.  Thus tzuras is a spiritual disease found only with talmidei chachomim