Nowadays, ill health is emblematic of purely physical deficiencies. For example, diabetes is symptomatic of a lack of insulin, and a broken arm hurts at the site of the break. Once upon a time, however, various spiritual malaises were also associated with physical symptoms. Just as pain is the body's inbuilt mechanism informing you to slow down for a while, or avoid using the limb in question till it heals, so, too, diverse spiritual ailments would present in a physical format as a symptom of the sufferer's moral, ethical or religious deficiency, and thereby grant one the opportunity to rectify the problem.
The most famous of such diseases, tzaraat (often mistranslated as "leprosy"), is extensively discussed in the Torah. Said to strike if the patient had indulged in gossip or malicious talk, tzaraat would present as various whitish spots that would break out on one's body, house or clothes. The Jew in question would visit a cohen to confirm that he was indeed suffering from tzaraat and then be sent out of the city to undergo an intensive program of repentance. Once the symptoms would dissipate he could begin the purification rituals and subsequently resume regular living.
While the Kohen wasn't paid for his services and thus couldn't be accused of padding his nest at the community's expense, the question still remains: why did they need a Kohen?
Were an educated member of the public to diagnose all the symptoms of tzaraat, the sufferer would still need to have the diagnosis confirmed by a member of the priesthood, no matter how ignorant. Even if said Kohen was acting under the direct instructions of said scholar it would still be the exclusive prerogative of the priest to formally pronounce the diagnosis.
Kohanim were entrusted with a sacred responsibility. Daily they would gather in the Temple to bless the nation. Ashkenazim commemorate this priestly blessing every yom tov (festival), while Sefardic communities enjoy a daily priestly blessing.
Kohanim have come to symbolize "men of blessing." Innately concerned with the benefit of the nation and attuned to their every need, they alone had the capability to render judgment in case of the occasional malefaction.
Hence an important lesson: Occasionally one observes improper behavior on the part of another. How tempting to stand in judgment, and to banish the sinner "out of the camp." From the Torah's insistence that the Kohen play a part in the drama, no matter how seemingly redundant his role, we learn that the only ones qualified to condemn are those who have served their time in the cause of love.
Only someone who has proven himself to be truly dedicated to the welfare of others can dare to criticize, and even then, only if he is willing to involve himself in the process of atonement and forgiveness.
-adapted from www.chabad.org
