When we develop a skin condition today, our first response is to see a dermatologist. During Temple times, the correct reaction was to find a kohen whose job it was to evaluate whether a lesion was a nega tzara’at – the leprosy-like plague described by the Torah.
According to Rabbi Meir in the Mishna, a person who suspected that he was suffering from tzara’at could visit a kohen on Hol HaMoed, but the kohen was only allowed to declare the nega to be healthy. If he found the person to be tameh – ritually defiled – he could only announce that after the holiday was over. The Hakhamim rule that a kohen could not check potential nega’im until after the holiday was over.
The Torah describes tzara’at as a condition that could only be evaluated by a kohen. When a person would see a mark on his body that he suspected might be a nega tzara’at, he would show it to a kohen who decided whether the nega should be disregarded, watched, or declared to be tzara’at. Even in generations when kohanim were not expert in evaluating the nega’im, they played an essential role. The trained Rabbi who examined the spot would offer his opinion about whether the nega was actually tzara’at or not. In any case, the person remained tahor (ritually pure) until such time as the kohen – basing himself on the recommendation of the Rabbi – would declare the individual to be ritually impure.
These rules are unique. In contrast with other laws of tuma ve-tahara, a nega tzara’at was not considered to be ritually impure solely based on the appearance of the lesion, but rather based on the statement of the kohen, which was essential. Without his statement, even a clear nega was not considered tameh, nor would a recovered metzora (one stricken with tzara’at) be considered ritually pure without the kohen’s official statement.
Since we are no longer certain who is truly a kohen, we no longer apply the rules of nega tzara’at as they are described in the Torah.