A doctor didn’t diagnose leprosy, nor did the examining priest. It was God who made the decisions!
It’s vital to note that the decision whether or not a person had leprosy wasn’t really the priest’s. The priests made careful observations, but it was God who gave them the diagnostic tools which they used. These were all yes-no decisions. If “the sore appears to be deeper than the skin of his body” (Lev 13:3), the answer is yes, it’s leprosy. If “the sore has faded, and the sore has not spread” (v 6) after seven days, the answer is to wait a further seven days. After that delay—which to the patient would no doubt seem interminable—a third examination took place. If there was still no change, it wasn’t leprosy; it was just a scab after all. Notice there was no rush to judgment, and so it should be when we are dealing with our fellow believers. Yes, sin must be dealt with, but remember the words of the Lord Jesus Himself: “Do not judge according to appearance, but judge with righteous judgment” (Jn 7:24). Jumping to conclusions based on superficial appearances can be as damaging as misdiagnosing leprosy then or cancer today. Notice the detailed layout of Leviticus 13. 1. The priest makes a careful diagnosis (vv 1-8), taking all factors into account; 2. What to do if the old leprosy problem flares up again (vv 9-17); 3. The distinction between a boil and leprosy (vv 18-28); 4. Discovering leprosy under the hairline or in the beard region (vv 29-39); 5. Leprosy and balding (vv 40-44) is described, where “his sore is on his head” (v 44); 6. How a leprosy sufferer should behave (vv 45-46), living outside the camp and warning any who might come near that he is “Unclean!” (v 45); and finally, 7. The effect that leprosy might even have on clothing (vv 47-59). How complicating both physical and spiritual defilement can be!